Tools for image editing/other means?

[Simpsons Dumper]

I use TexturePacker. The free version works just fine - the pro version organises stuff optimally for compression with game engines, which you don't want if uploading to the VG Resource websites.

If you're on Windows, you can use Daxar's Sprite Sheeter. Judging by the screenshot, it looks like it'll get the job done.