Images should be resized before they are uploaded
If you don't have Photoshop, there are a number of free options for editing images (resizing, cropping, etc.).
Because BeFunky has decent video tutorials, they are referenced here.
BeFunky, Pixlr, and Fotor have very similar functionality.
GIMP and Canva are a bit more complex.
How to Edit Photos BeFunky
How to Save Photos BeFunky
Basic Size Guidelines
Maximum content width
This is the maximum width of the content area. Images can be wider than this, but should not exceed recommended file size.
Should not exceed 1MB. 500kb should be large enough for most images.