Lots of 'Validated" (EPUB+CSS) things choke older RMSDK devices
JPGs as big a apple covers (I fixed a book that had graphic Chapter numbers larger than 800x600

so it would also open on my Astak. It was scaled smaller in the CSS

why ship that size )
GIFs even close to a physical screen dimension
And, as been mentioned: the 260K boundary (s/b smaller if using larger images on the page)