Hi Rachael,
Currently there's no direct support for that in GWD. We're hoping to get frame by frame animation by setting the background-image in the CSS panel for each keyframe soon so stay tuned for that! Currently you cannot use CSS panel to modify the properties of the element on a keyframe.
You can also convert your video to animated gif and use it as an image in GWD.
For now, there's a way to achieve frame by frame animation with a little bit of hand-coding required but it's super easy to do. I'm attaching the file created in GWD so you can check it out.
Here's how the workaround works:
1. In the Timeline Advanced mode, draw a div and insert a keyframe for each PNG that you'd like to use. You might want to put the keyframes close together or play around with the duration to get the animation close to a video.
2. Put all your PNG in the same directory or relative to the HTML.
3. Switch to code view and edit each keyframe to add
background-image: url(image.png);
4. Preview and you should see the frame by frame animation play PNG sequence.
Hope it helps!
San
GWD Team