A lot of image hosting places resize the photos as part of the upload process. Depending on what tool they use to resize, and what quality level they save at will determine the quality you actually see out the other side of the conversion.
You'll notice your two pics are very slightly different dimensions, and also different file sizes - the second/photoshop version being 40k bigger.
I use http://www.filedump.net for posting stuff on forums. It only accepts one file at a time, but its quick and easy to use. And since it just hosts the file, regardless of what it is, it doesnt play with the image quality on the way through.
EDIT: If you want to examine the differences between pre- and post- photobucket, grab the Exif Viewer addon for Firefox. It lets you right click any image and do 'View Exif Data' which is all that hidden stuff saved in a photo, such as quality setting and camera details :rolleyes:
