If your GIF file is not playing or looping, it might be because the file is too large. If it’s more than 1080 pixels high or 1920 pixels wide, you’ll need to reduce the size.

There are a number of free, online tools of varying sophistication that you can use.

Here’s a simple one that will do the job: http://gifgifs.com/resizer

Here’s a more sophisticated tool with additional functions: https://ezgif.com/

If you have a large (in MB) GIF animation your iPoster can take a very long time to load. If so, you can also use this tool to optimize the image to reduce its size. Try a setting of 100%.