To decrease your data storage, you should use the minimum size for your image. You can use the below size:

Header images: 1841 x 514 px.

Course, blog featured images: 870×580 px.

Slider Revolution on the homepage: 1920 x 1280 px.

Category icon images: 64 x 64 px.

Background image heading for category: 150 x 100 px.