Image File Sizes
Facebook requires your images to be no more than 100kb in size. Optimising your images for the web can be done through photo manipulation programs. If you don't have Photoshop then the free GIMP program will be very useful. Keep your photo dimensions above the display settings to avoid photo stretching and distortion.
Image File Types
In general photos will be better saved as .jpeg/.jpg files, whereas illustrations, icons and other graphics will be better as .png files.
Any image text will need to be centred for it to be completely read on both desktop and mobile.