Pics should be fine, as long as you can find a way to convert them into WebM videos first. In fact, if you can make them fade into each other, it would probably look very nice.
Unless a patch changes the videos, or you verify your game cache in Steam, the changes will be stable.
I create an slide show with powerpoint (with fade effect) and export it as a video file (mp4) and then convert mp4 to webm (VP8) with Pazera WebM Converter, result is good.