GIMP works, but Photoshop is a little more flexible with it, as you can open your layers in the Timeline and set them as frames, then adjust how long each frame lasts, if needed. So it just has a few more options when it comes to making them.
Basically, when you make an animated gif, just treat each layer as a "frame". This means that if you were doing an animation of, say, a twinkling star, with two looping positions for the star - you'd need to put the same image twice on separate layers, and then change the animation on the second layer above the first, so that when you go from one layer to the other, it looks like it's changing, and then in turn when you animate it in Photoshop/GIMP, it loops on its own.
Also, the size limit for gifs on Tapastic is the same size limit for images on Tapastic, as you upload them the same way you would as an image - 2 MB, with a maximum width of 940 px.
