1. To often I have problems like these. What I feel is wordpress is trying to resize your image again. While on the web it works fine but on the phone app. This is sometimes what happens.

Solution seems to be

basically after it is working ok. Sometimes you still get errors because perhaps you trying to type your post while uploading images. Somehow it seems to cause this. Alot of the images without thumbnails are corrupted images and to clean up you can delete them.