My K4 (e-ink) and my Kobo Aura2 both display images.
True, they are grey scale, but they are displayed. Maps are visible
SAVE SPACE (And you don't claim to read Manga) ???? most (~65%) of my library is made up of novels under 400K.
Very few go past 1M and those tend to have embedded fonts and additional images (Image only Chapter heads/numbers. your plan would destroy the book).
With 4-8G of device storage, I have almost 100 books (almost a half-years reading) and not even using 1/2 the storage.
OTOH I do reduce HiRes covers to 90% of my screen size (600x800). That saves PROCESSING (scaling to device screen) using the resize cover plugin (this changes the Library copy, which will be also become embedded on demand)
Please reconsider your options or you may end up with a lot of damaged content
when the cover is not the only image 
This is not to say that the use of the image tools in the Calibre editor should not be SELECTIVELY applied
Nor the removal of
excessive advertising (Authors: do we really need HiRes covers from all your published works included? Forthcoming
)