Do you have some idea to manage the size image.Is it better to use jpg, png ?

I think this element could be very important :

If the background is blank, it's better to use jpg because the weight is low.

You must work with the good size of your image with no auto resize in function of your page

 

 

There is some tools on Internet can optimize the image for you. You can use it before to create your product.

I think there is a hook than you can install and make an optimization when you create the product. If I remember me, it's Image on the fly, the name you can find inside the marketplace.