Yes.
We recommend uploading images that are 10mb jpegs. This is typically enough for an A3 double-page magazine spread.
BowerBird also makes a smaller version for web use that is 1200px wide.
The largest files you can upload is 20-50mb. (Any image that takes longer than 30 seconds to process may time out).
You can learn more about uploading images to a BowerKit here.