You choose a color depth, it should be 2, 4 or 8 to make life easier.
How do you normally send the image to the display? Byte by byte serial or byte by byte parallel?
Either way, the display only gets sent 1 byte at a time. Figure out how fast you need to have them ready. Pulling data from flash takes 1 more cycle than from RAM. You need to supply 16384 pixels but you don't need to have them all in RAM at the same time. Don't expect PC graphics speed even with a high-end ARM board. AVR is slower, expect that and get what you can.