Last I checked, GIFs work perfectly fine with [image] tags. Source: any time I wish somebody a happy birthday.
In terms of uploading GIFs, if you try doing that on the standard upload tool on 3DS Paint, it'll only show the first frame as a static image, as I'm sure you're aware. Likely done to not hog up memory on Hull's servers.
HOWEVER, there is a workaround. If you upload it as a profile picture, you can use that as a image; just use the [image] tag.
If you want to see some examples, just head on over to my DSi Gallery. I have plenty of those there :p