This is the only Homeworld (2) GIF you'll find, since I made it, a long time ago actually:
Vaygr HAC-IIB fighters from the PDS mod, so don't worry if you don't recognise it.
Even if that was 130px wide it'd still be over 300KB, no way it'd be usable here.
Capture a video (but first turn the game HUD off) using Fraps or some such program, open video file (.avi) in Microsoft GIf Animator (Google it, not too hard to find, and yes it is freeware), cut lotsa frames out until the animation is about, say, five frames per second, save it to wherever you usually save things.
Resize the image using GIMP (unless you have Photoshop or something already; again, Google it, it is also freeware); be sure to resize all layers, in the case of this site, to the largest dimension being 130px.
Even once you've done all of this though, you will find that the image size limit on this site is atrociously low for the pixel dimensions they allow, so your avatar would have to be something like 80px wide, and at such sizes it's really not worth anyone's effort; watching dark spaceships fight in a dark space in a very small box beside a post on a forum is... pointless.
In the end all I can say to you is not to bother, the most frames you can put up in an avatar on this site is about six, and that's not even enough for me to use any simple 100x100 animated avatar I've found on any other site.
holy crap, when can printscreen and MS Paint make animated stuff happen!?
Take note of the website that 非常に高いレベル mentioned.