Sometimes it can take a while, took mine 30m T~T But if it doesn't work you could save the picture and upload it as your profile picture, just instead of clicking gravatar you click Browse (The first option when you change your profile pic)
Gravatar usually takes a few minutes to update, also, make sure you've connected the correct Gravatar account to your forums account. You can click 'Test' in the Gravatar option when choosing a profile picture to sort of force-update Gravatar although that doesn't always work.