Importing an Image as a Set Symbol
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.