Images - Interpreting Results

Please could I have some help in interpreting the results with what is going on with the images on my website.

Generally my images are nearly all the same size (landscape) and optimized.

Therefore I am not understanding why some are taking 100ms to load and some 3000ms to load.

Is this something I need to correct ?
The test is done on loading my homepage where the images all end up the viewed at the same size.
Why the discrepancy ?

Does there happen to be anything else where I could speed up my website with easy changes.