IMDb is implementing a new design (summer 2020). Depending on your cookies, you are given one of the 3 layouts.
Old design (pre-2020)This userscript adds a stylesheet to remove dead space around the image.
Note that even if your adblocker hides the space used by the ads (the "dead" space at the top of the screen in the screenshot), it doesn't make the image use the whole space. This stylesheet does.
Design June-July 2020The stylesheet has been updated to work with the new design, and make it look like the old one: hiding the header (menu and search field) and the footer of the page and recovering the space used by the top bar (back button and # of image).
It toggles the display of the top bar like the bottom info box, which isn't done by default.
After installing this script, reload imdb's page.
Old layout
New layoutbefore:

after:

You can also download this as a userstyle version, but the top bar stays visible. (get it from
userstyle.org)
For more userscripts, styles, extensions, tips, ..., visit my
programming page.