You must understand that image "dimension" size and image "weight" size are very different things. If you plan to display fullscreen I wouldn't go below 3000x1500 pixels in dimension size with 4000x2000 better suited. Allowing that people have widescreen monitors these days running 1280x1024 resolution commonly.
A fullscreen image at 4000x2000 will typically run you at 800kb to 1.5mb each if compressed in "weight" correctly.
Programs such as Photoshop's "Save for Web" feature or KissMyImage software do a good job at compressing images so that the weight is minimised.
KissMyImage has a great feature where you can input what is the "allowable" target weight for the compressed image.
As always, some images will compress better than others and it is always a trade off between download speed and display quality. Use your eyes as the best judgment - if it looks bad it's most likely over compressed or the image dimensions are too small for fullscreen display.
Remember, just because you "can" doesn't mean you have too! Some images just shouldn't be shown fullscreen...
Regards, Smooth 