title_body: Alternate covers for different devices and software Quite new to writing and editing ebooks and I wondered something.
  I know in webpages it's possible to produce alternate styling and content based on what the content is being viewed on. So CSS can style something differently for print than for screen and differently for mobile than for tablet or PC.
  Is the same true in epub?
  I'm thinking most specifically of cover images and other (potentially) colour content.
  I'm painfully aware, due to being sight impaired, that some book covers which look great and are easy to look at or read in colour are less than optimal in black and white or are great as a full size cover but barely recognisable as a thumbnail.
  Is it possible to code an epub so that a different cover image is shown on a black and white ereader than would be seen on a colour screen.
  I'm asking not just as a user who needs acessibility but also as someone who would like to produce ebooks which are as acessible as possible.
  The other route is going for high contrast low detail colour so that, monochrome or coloured and large scale or thumbnail, you always get something recognisable and readable. However this places more limitations on design options.
  Of course I'm not even sure that thumbnails are a part of the epub or whether they're just generated from the main cover image by whatever software or device holds the ebook.
  Any advice on this would be greatly appreciated.
upvoted_answer: Generally the answer is no. You are defining  only  one cover in your source -- but you can easily test how it renders in both color and greyscale. So it should be high contrast and not so much color-dependent.
  The next question is required resolution and file size. Generally, Amazon and others require one high res cover but will downconvert the file size depending on the display size.
  I'm not an expert on accessibility (and am certainly no artist), but know a lot about formatting and what is possible  for ebooks.  The key thing is reducing the key elements in the cover -- unless you are going with something that looks like an actual scenic view. Even that is hard to pull off (although in scifi, it is more commonly done).
  By the way, from a marketing perspective it is crucial to have only one cover for a ebook. Even if a second cover is a simplified view of the original cover, it can confuse customers and ebook distributors.
  One thing you should absolutely consider is how the cover appears in a thumbnail. Full scenery in a cover often does not work well in thumbnails. I wrote a little piece about ebook covers and noticed these constraints on ebook deal ads.
  
  One last thing. A common way to advertise your ebooks is through ebook
  newsletters. Each daily newsletter normally brings 20+ book ads with
  the ebook cover. The art included with these newsletters tends to be
  small, so you should make sure that a thumbnail conveys the cover
  concept adequately. I haven't verified these dimensions, but a quick
  check in my browser reveals that Bookbub uses 215 pixels, Bookgorilla
  uses 180 pixels, Bargainbooksy uses 250 pixels, and Fussy Librarian
  uses 170 pixels...Typically on Amazon, the ebook covers appear as
  150-175 pixels wide on the search results page and 300 pixels wide on
  the ebook product page.