Seel Adafruit’s tutorial. Also consider BMP files.
Looking around it seems it takes 12msec to do a 512B block read.
This post claims it takes 1msec.
That’s quite different. If it’s 1msec, it means I can read the SD-card while drawing pixels. Otherwise it means I can’t and I need to wait for the refresh, which limits the display to the amount of memory available… I’ll have to experiment!