Can someone help me with my GIF problem?

I have made multiple GIF pictures using Sony Vegas Pro 10 and Photoshop CS5, and everytime it works, like, on my computer, but when I go to post it online on Tumblr, the image doesn't move and is frozen. Does anybody know what it could be that I'm doing wrong? I tried saving it as images and html one time, and that still didn't work. Please help if you can.