If you don't want to use the image handler there's another way optimise you images, similar to having extra images per product.

Zen's auto scaling can result in images with less than optimum files sizes and thus consume bandwidth.

If you want to show your images with the relevant images sizes you need to provide additional information to Zen Cart, and it will recognise the images by there suffixes of "_LRG" or "_MED" respectively(large and medium)

If you go to yourwebsite.com/images you'll see the directory’s called large and medium will be there.

If you create your images like below and ftp the medium images to medium folder and same with large

so you should have
mobile.jpg (small image"base image")
mobile_MED.jpg
Mobile_LRG.jpg

You can configure the size of your medium images and large images in

Configuration-> Images
Product info - Image Medium suffix
Product info - Image Large suffix

If you have photoshop it would be able to do very quickly with actions

Hope that helps, I did a Zen Cart store 2 years and am giving it a complete overhaul at the moment that's why its fresh in my mind