My gani editor already lets you select the image area you want to add
. It also has an interface for attaching sprites to each other, but I don't like it and it needs work. I haven't had much time to work on it lately due to school though, and I need to refactor some parts so alpha blending can be shown in previews (already works on the main editing canvas) and so exporting to GIF can be done.
(old screenshot)