Complex sentence: The practice gained prominence in 2013, primarily through the websites Mediacrush and Gfycat--websites that allowed users to upload GIFs to be converted to videos that are displayed in the browser similarly to regular image files.
Split into simpler sentences: The practice gained prominence in 2013, primarily through the websites Mediacrush and Gfycat. They allowed users to upload GIFs to be converted to videos that are displayed in the browser similarly to regular image files.