There are several ways to do this ....
You can login by the following link as this site can change the size of images online and downloaded on your pc or laptop and then upload it again on your store
Also there are several programs can be installed on your pc or laptop then control the size of images and upload again on your store with the same quality picture, including the following:
Fotosizer on the link below download link the program with an explanation of how to use it
There is also a program called Snagit you can download this program through the following link also I will put serial number of the program as its require when setup it