Been working on some apps, and I was using the same naming convention for the images. logo.png for the logo, and screen#.png for the screens. However, they all shared the same logo, because it looks like it puts the images in sites/default/files/styles/apps_screenshot/public/apps/file.name. So all of them are referencing sites/default/files/styles/apps_screenshot/public/apps/logo.png. Each app should probably get its own folder in there.