Ok, so everything you've tried so far really should have worked. Especially erasing all the pictures. My best guess is that something in the custom scripts you're using is causing the erase picture function to not work as intended. If you can, post a link to the scripts you're using particularly the "map as battleback". In the meantime, try the following to help diagnosis the problem.
1. In a battle, use events to show a picture on the screen, and then erase it. Exit the battle, and re-enter. Does the picture show up?
2. In a battle, use events to show a picture but don't erase it. Exit the battle, and re-enter. Does the picture show up?
3. In a battle, use events to show a picture but don't erase it. Replace the picture with a different one and erase it. Exit the battle, and re-enter. Does the picture show up?
4. In a battle, use events to show a picture but don't erase it. Replace the picture with a different one but don't erase it. Exit the battle, and re-enter. Does the picture show up?
Just so you know, based on what's happened so far, I'm expecting the answers to all be yes. But when I do these on a fresh project, I get "No", "Yes", "No", and "Yes" respectively which is partly why I suspect it might be some custom scripts.