@mafein style tweaks>User style tweaks>"Add your own tweaks in koreader/styletweaks".
Sounds you have some pretty badly formatted ebook. Either you try different software with their default settings or you code your own styling.
Sometimes it helps a lot to open the menu and select the lower menu>gear tab>Embedded Style>off.
After that you start tweaking the styling. E.g. style>HTML5, styletweaks>Full-width images.
If that's a bit too much, why not try double_sized_img.css
Just Google on 'img', 'html', 'css'.