Do you mean that you and others would edit at the same time or independently?
If it's independently then there's loads of online photo editors like this one. Alternatively, you could use GIMP, Photoshop or a different one.
You can try SumoPaint. It's an online software. It requires flash, so when you click "try online" make sure you go to the left of the address bar, click on the lock, and for flash, choose "always allow on this site".
Gimp confuses the hell out of me. I got used to photoshop, but I only use a photo editor for super basics, cropping/levels/contrasts/sharpening... I've found Sumopaint.com to be more similar to Photoshop and it's nice to be able to use it in a web-browser, not having to download anything.