This seems to come up quite often on the site, and I felt we needed to make a thread on it that all members could find, and use for reference.
When uploading pictures, profile pictures need to be in .jpg or .gif formats only. They need to be around 300x250 in size. If your profile picture does not change, once uploaded, hitting CTRL-R should refresh the page and show the new, updated picture on most browsers. However, there seems to be an issue with Google Chrome browsers, specifically, that require you to clear your cache and browser history before the picture will update to the new image.