Hi Tom
As @Bettye6 mentions, the usual culprit is the pictures must be at least 1024x683 pixels, or the photos are not accepted. If you are trying to upload from a phone, this could also be the issue. Phones will reduce the quality (pixels) of photos to allow for more storage on a phone. So if someone texts you photos, they will be a lower quality.
It's best to email the photos to yourself and download them on a computer. Then upload to your listing from a computer.