Anybody help me? Thank you very much!
I can only tell you what I do and I don't have problems.
I first size my JPEG to fit the space I want. I then remove the metadata to make it less 'meaty'.
I use the INSERT Tab to add the JPEG, not copy and paste.
You cannot ensure that an image will be full screen in the hi-rez devices unless you upload the book as an html (or mobi or epub) file, and then only if you put a width="100%" instruction in the html. However, the reader in some devices can tap on the image to make it zoom.