i've done some test. here the results
1: every image in the first file (the cover file after conversion with calibre) is displayed in full display.
2: inserting images without the svg and with only a simple <p> tag doesn't work.
header and footer are present and the image gets downsized.
moreover from what i could see in another test, after switching the cover file with another file, the latter one will be displayed in full display.
so there isn't some particular code in the cover file that tells the device what to do but is a command in the FW that makes every first file in every kepub to be displayed in full display.
that said it seems the only way to get other images (besides the cover) to be displayed full display are:
1 put all the images in the cover file
2 create a class only for kepubs (like the koboSpan) to be added to the images and recognized by the device for a full display rendering