Online Image Editor (OIE) Animation Wizard Index, which lets you upload at least 2 images (max 250x250 pixels each, but they can be different sizes) to create an animated GIF i.e. an animated image which fluctuates between the uploaded images. You can even choose how you want the transition between the images to work.
There's the size limit, and it's slightly erratic in that sometimes it doesn't work (probably when it's very busy), but hey it's free. Handy for e.g. avatars for bulletin boards and forums if you want to have an animated avatar, though in that case you may need to further reduce down the size of the pic to fit in with any image size restrictions imposed by the forum.