Images are importable for the set symbol editor. Start by opening the symbol editor, then choose File -> Open, and change the file type to images. Navigate to where you image is saved. Yours should be visible; just open it in the editor and proceed.
Now, some advice on how to get good results when importing images.
1.) Only use pure black and white images. Other colors (even if they look like black and white) cause ugliness in the symbol editor in the form of aliasing, no matter how close to black and white they may be. Black is where there will be symbol, white is where the symbol will not be.
2.) You'll also want the image to be 100% Opaque. Transparency leads to large unsightly blocks in your symbol that you have to edit out.
3.) Use a file format like png, something that does not compress when saved. Jpeg does not work well for this.
4.) The larger the actual image you import, the better. The bigger that image is, the smoother the curves become inside of the symbol editor, and that's always a good thing.
5.) More square images are better, because they fit in the editor with less distortion. This happens because the editor stretches (or compresses) the image to fit in the editing grid, and the aspect ratio is not locked. So it may stretch or compress more one way than the other, distorting the symbol. But the editing window is close to square, so the image is best as a square as well.
6.) Be aware of how the image is sized. If you want a longer symbol (like the Urza's Saga symbol), the editor can do that, but you'll have to leave more white above and below the symbol to compensate (and to account for what I mentioned in step 4). So the entire image should be square, but the black part inside should look like what you want your symbol to look like. When importing into the editor, the white field can affect the shape, but after that, all that empty field is cropped off when placing the symbol on your card.
7.) You'll probably have to do a little after import smoothing using the symbol editor's tools.
The vector editor itself has five main functions: drawing shapes, moving shapes, rotating shapes, altering shapes, and creating symmetries. Before ever working on the editor, I will normally grab the right hand side of my window, and stretch it until I can see the entire top toolbar. Even if you think you can see it all, make sure by stretching the window a little farther.
To draw a shape, choose "Basic Shapes" on the left toolbar, then choose a shape at the top. If you choose a polygon or star, make sure to choose a number all the way to the right. For polygons, this is how many sides, and for stars this is how many points. When ready to draw, click and hold in the editing grid, and drag in some direction, then keep holding and dragging to manipulate the dimensions and location of the shape. If you mess up, let go of the mouse button, click delete, and try again. You can also select it to delete it, which I'll talk more about further down.
The second function is moving shapes. This is done by clicking select on the left toolbar after you've created one or more shapes. This allows you to grab the middle area of a shape and move it, grab the handles shown on the sides, top, bottom, and corners of a shape to resize it in one or all directions, click delete to get rid of the shape, or to choose a shape type on the top toolbar. Shape types determine how the shape looks. Don't be afraid to play with these to see what they do; its as easy as clicking another style to change. All shapes are Merge shapes by default.
Merge - Will make two shapes look like one by coloring both the sections that overlap and those that do not.
Subtract - Makes one negative. This means the shape will not be visible on the cards, and in fact, will hide areas where it overlaps with other shapes.
Intersect - Only colors the overlap in two images; will look like Subtract with one shape, but will be different with two, especially if both are set to intersect.
Overlap - Will make the image that is higher in the shape list overlap the ones that are lower in the list. The shape list can be seen on the left under the left toolbar.
Border - Makes one image look like border, allowing the border of another shape to be extended.
The third function is to rotate shapes. Select a shape by using the Select tool, then click "Rotate" on the left toolbar. you'll see two-headed arrows on all four sides and all four corners of the shape. The ones on the four sides will stretch that side of the shape in the direction you drag. The ones on the corners will allow you to spin the shape the direction you drag.
The fourth function is to alter shapes by manipulating their points. First select a shape using the Select tool, then click on the Points tool on the left toolbar. Now you're looking at the defining points of a shape. Rectangles, polygons, and stars have a point at each corner of the shape. Ellipses have them at the top, bottom, and each side by default. You can grab a point to move it, or click on it once then click delete to delete it. You can create a new point by double clicking on a line between already existant points.
If you grab a line, you can move it and its two end points at once. After clicking on a line, you can switch between "Line" and "Curve" on the top toolbar. "Line" makes it a straight line, "Curve" gives you two handles on dotted lines that you can use to manipulate the shape of the curve.
If you create a point on a curve, it has curve handles of its own, and can also be set to "Free", "Smooth", or "Symmetric" on the top toolbar. "Free" allows you to move the two handles that appear around the point however you want. "Smooth" locks the handles in at 180 degrees from each other, but allows them to still be stretched differently. "Symmetric" forces both handles to be the same size and length.
The fifth function of the editor is symmetries. This is a more advanced tool, and the most recent addition. Select a shape, then click Symmetry on the left toolbar. Now choose "Add" on the top toolbar to add a symmetry; click "Remove" to delete the symmetries. Choose a number on the far right of the top toolbar to decide how many items should be in symmetry. Symmetries can not be reshaped on their own. They can only be manipulated by altering their symmetry properties, or by altering the shape that they are a symmetry of. Symmetries can be rotations or reflections, and any shape can have any number of separate symmetries.
*Note: There seems to be a bug I've found in the editor that causes the Rotation button on the top toolbar to cause a Reflection, and the Reflection button on the top toolbar to cause a Rotation. For this information, when I say Rotation or Reflection, I mean which button is pressed down on the top toolbar, not what it actually does.
When working with Rotations, you can click the center dot on a symmetry line to to move the symmetry around and reposition it. While working with Reflections, you'll get a handle on a dotted line off of the center dot. Grab and turn that to choose where the symmetry appears at. You may use a combination of these tools to both turn the symmetry and change its location on the grid. If you're having trouble controlling symmetry during Reflections, move then handle further away from the center dot for more precise control.