You can go about this two way. If you go the spritesheet option you'll have to place the 4 icons unread, read, page and link icon onto one sheet, link to them within those options vonDoom mentioned inserting the coordinates (you may be able to use existing coordinates if your custom spritesheet is the same size, if not you'll have to adjust the coordinates to compensate for the new sizes if they happen to differ.
Or use individual images. Upload your images to the server as normal and simply link to them.
Yes to your question though. Overwrite the existing spritesheet or create a new title on the offchance you want the original as a backup.