Hey
The current live version is not allow you to do this. Use an icon is almost impossible, this why the new version will have a completely modified front-end part. (Don’t ask for ETA)
You can change the text, but it’s not easy. You need to modify the sourcecode of the plugin.
You need to find the following part of the minified javascript and change the text:
https://github.com/SubZtep/gif-animation-preview/blob/master/gapplayer.js#L181
If you just add one more character (for example ‘play’) it will work, but style will be broken, you need to fix it too. For 4 characters long text this will be the right css:
https://gist.github.com/SubZtep/b2c35812f15d672a089e
I hope it helps, if not, you need to wait until the next major release.