I wouldn't know what happened without seeing how you did it, but the way I do it I use the following steps.
At the start, you can use any gif or short video file format (like giphy, gifv, webm, etc.). Next, use an editing tool to get the gif in the form you want it. I use ezgif.com. I also crop and edit them so that you have a 1:1 aspect ratio (the tiles in the game are squares, so anything outside the square is cropped). Then, and most importantly, MAKE SURE IT'S CONVERTED INTO A GIF. The game site only accepts .gif files, so if I had to guess, you may have upload a gifv or webm format. Save the .gif file onto your computer, then upload it to your game. Good luck!