What JSWolf mentions is true about the screen size/resolution...Just a little clarification, you are not required to fill the whole screen. Just make sure your images still look good/acceptable with the higher resolution screens. A small (300x500 pixels) image at "72DPI" (although DPI is a printer resolution, not truly a screen resolution) shown on an 8cm wide screen at 300DPI would look incredibly small.
Also, make sure you are testing it on your target devices/readers. If you are not selling on the Kobo or Kindle market then there is no need to cater to their peculiarities. Having said that, you'd be cutting yourself out of a HUGE market share if you didn't sell there!
Cheers,