Your computer stores a copy of all the pictures on the site that it downloads. This is so that it doesn't have to download every picture every time you visit it, making the site faster. It only checks to see if the pictures have changed every so often, and if they have, then it downloads a new copy.
You changed your pic on the site but your computer hasn't bothered to check to see that you have. Other people (whose computers check on a different schedule) already see your new pic.
Clear your cache (as SR113 described), and it will force your computer to re-download all the images. Or, in many cases, just press 'F5'. Or just wait a couple of days. It'll catch up with itself.
Work, drink, sleep, repeat...