When optimising your zencart install there is a very powerful plugin available. This is Image Handler, it comes in four (to date) variants. Version 4 is for Zencart 1.5.x, whereas version 3 is for the 1.3.x versions (still the most common around to date).
What does it do?
basically it resizes images on the first load so smaller ones are then used when required, images generated are then stored in a cache so your system doesn’t have to redo them.
The advantages of this is loading time, for example a test image was 14Kb, the resized version was 5.7Kb, so you can imagine when you are loading 12 images the saving and speed-up is quite substantial.
Your full size images are still available, and are used for the larger image view clickable on the view listing page.
To give you an idea – the implementation of this plugin – has reduced the load times of the store I worked on by at least 50% on the main page, more on results pages where 12+ images might be loaded. In addition the bandwidth consumption has dropped around 25% (despite an increase in engagement of visitors).
CPU doesn’t seem to be high, the first creation takes less than a second (so on day one page load times did increase slightly – however by day two they had reduced and have continued to fall as more of the images are created).
If you want to see it in action visit Delicate Dreams (Sterling Silver Jewellery) and check the attributes of the images (and the sizes).
One other addition with this plugin is that you can have a hover – this gives a zoomed in view of the item – particularly useful for jewellery or ornate items.