The most popular image formats are supported, such as PNG, JPG, and GIF. Image formats such as BMP, however, is not supported.
When you prepare images make, sure they are as close as possible to 1920x1080px.
Screenly OSE (and the underlying video, omxplayer) is pretty picky about video content. In order for videos to be properly played, they must be encoded as H264 MPEG4 videos. If not, it’s likely that the videos won’t show up at all.
As long as you properly encode your videos, Screenly OSE can handle 1080p videos beautifully.
In addition to regular videos, Screenly OSE also support RTSP feeds, which are commonly used for things like surveillance cameras. Please do however note that the feed must be h264 encoded. You can learn more about that in this blog post.