In edit mode, when over boxes, click edit to edit section
Then click edit over a specific image
Then remove image and re-upload new.
All body images work well when sized to 1,500 to any size high.
Name the file with good SEO keywords consistent to content and then add animation control (if applicable) to the end of the image file name.
BONUS POINTS:Run the image through Image Optim to get the file size down maintaining good quality. PNG or JPG.