I made a gif on PSE10 from a movie clip and when I open it from my computer files it works fine. I've saved it as a gif but when I post it anywhere it just shows the first frame frozen. Any ideas why?
That's just too massive for an animated gif to work well, since it must download completely to the computer of visitors to the web page before the animation starts, and it's going to be a longish wait for most connections.
Use smaller images at a lower quality level, use fewer frames. But a gif is not capable of substituting for a movie. An average size for a animated gif would be less than 100KB, way less. You can't make a short movie clip work as a gif. Two frames maybe, like a before/after kind of thing, but much over that, you're really pushing it way beyond what it's meant to do. There's a reason the animated gifs you see on websites are mostly pretty tiny.
I forgot to mention that this is also a question both of the server's capabilities and the viewer's connection speed. Here's an example of a large gif that loads pretty quickly, because the Washington Post has a lot of server capability and probaby not many people viewing a page about an exhibit of animated gifs:
The apple gif is over 3 mb but it opens right away. That same image on a site like, say, dpreview.com, would take minutes to load.