The animated image format is gif. There are many places on the web to download already made animated gif avatars for use on forums like HTG, but if you want to create your own you need to use an image editor GIMP is the one I use. To get an animation you put each frame of the animation on a different layer and save as a gif GIMP and probably other image editing software will ask if you want to save a flattened image or save the gif as an animation. You then can accept the the defaults or change the speed of each frame transition and whether you want it to play only once, twice, etc. or loop forever. Recently I was involved with a community video production and I made a full-screen animated gif of a hospital ekg monitor for a hospital scene (the only convincing ones available on the web were paid-for video loops and the budget was pretty much zero as all the participants were volunteering, so I just made my own).