Thanks, feb, carefully followed your instructions, but came to a halt after you said click "customise this folder". Couldn't see any "nexts" to continue, so didn't get as far as "image preview".
Had a good click around all the options, and the only way I've found to get it to show a moving animation with the folder open is to set the folder type to "Pictures (best for many files)", then under View, click filmstrip. But then only about 6 gifs are visible at any one time (there are dozens in the folder - yes I know, sad isn't it?!!).
On the old computer, I could have a screenful of gifs, hover over one, and a moving thumbnail would appear. Not any more, it seems.