Is this a problem with Gimp or with Photobucket?

Suddenly if I save an image as a .png it looks fine when I have it open in Gimp. However, when I upload it to the web (on photobucket) it shows up very dark and discolored.

However, if I save it as a .jpg, it does not seem to have the same problem. Where should I be troubleshooting? Is this an issue with photobucket, or does it have something to do with how Gimp is saving my images suddenly?