You can do it using Image Viewer... I did..... =/ Just export all the Spell Images you want to edit or change and paste your image over the top of them. Make sure the back grounds black before saving your picture. Then you will need to Import them all one by one, replacing the old spell.
Otherwise you can use the wileditor to move and replace the images to allow you have more area to work on your image/spell, but doing the samething above.
If you need anymore help just post back on here and I'll try explain in more detail.