There are several ways to do this, knowing that the maximum allowed image size is 2MB..
You can use many websites to change the size of images online and downloaded on your pc or laptop and then upload it again on your store, here's a list of some of those website:
Also there are several programs can be installed on your pc or laptop to modify or resize images and upload again on your store including the following:
Fotosizer you can download it from here:
FastStone Photo Resizer you can download it from here: