Hi Chris,
Ableton dropped support for image files in clipslots in recent versions of Live (it worked in Live 9). However, it still works in eSampler and eSimpler for png and jpg files. You cannot play a sequence of files though.
I don’t know what an easy way is to convert png or gif files to a video with alpha support. Prores 4444 is also an option for a video format with support for alpha which works will wit EboSuite. I will take a look what the options are and report back. EDIT: do you have access to Photoshop? alternatively you can use the free software Blender. I used this in the past. I hope it also supports prores 4444 or hap alpha (both of these codecs first need to be installed before they show up).
Btw, a temporary workaround might be to play with GifToSyphon which lets you stream gifs to a syphon source (also from the web).