Problem with images in mobile view

I’m working on an ebook currently and using Scrivener’s “novel with parts” formatting for it. I have small images before each part as a sort of introduction to each section and for the most part these look good, but I notice when I export as a a .mobi or epub and pull it up on the Kindle app on my mobile phone, the images look all wrong. They no longer have transparent backgrounds and are much too large for the mobile screen. However, when I test view the pages in Kindle Previewer they look fine. Any ideas on why this is happening? Thanks!