Unlike image file formats such as bmp and jpg, the "gif" format has a unique trick up its sleeve. It supports animated images, and most web browsers will happily display the animation as standard. So, for example, if you upload an animated gif file as your profile image or avatar on a web-based forum, or even Twitter, everyone will see the animation.
Creating animated gifs isn't always easy, but there's a handy web site that will apply one of a number of selected effects to the digital image of your choice.
To get started, head to http://www.viscomsoftonline.com/online-app/gif-effect/Online-GIF-Effect.html and upload an image (it doesn't even have to be in .gif format). Then choose your effect from the ones shown. Note that the finished results are generally neater than the crude headache-inducing animations shown on the home page of the site.
When you're done, choose an image size and your animated gif file is ready on your hard disk for you to use.