Hi Don,
I'm not sure I agree that changing the variable name would help out in this situation since the images on the product listing page are the same images displayed on the product info page... In other words both pages are asking for the same image, and that image is the one uploaded by the AIS which was told to make it 56 by 75.
As far as I know html will happily shrink and image to smaller dimensions (I think we've all seen those slow loading images that have obscene dimensions like 1600x1200 crammed into a 400x300 space) but it won't blow the images up to larger dimensions.
Since you're asking the product listing page to take a 56 by 75 pixel image and create a 160 by 300 image from it, it won't happen.
I've looked over both zen cart's and AIS's code and have an idea that may work but I haven't personally tested it. In your AIS configuration I think you should change the dimensions of the thumbnail image to 160 by 300 then add an image to a product. This will create an image with the proper dimensions you wish displayed on the product listing page. After you've uploaded the image then go back to the AIS configuration page and change the dimensions to 56 by 75 so that when the page's code is called it'll grab those values for the html the php is writing. It'll check it's variables and see that the image dimensions should be 56 by 75 and since the image it has is larger the browser should happily shrink it down.
On a side note, a 56 by 75 image doesn't have the same aspect ratio as a 160 by 300, you'll get better results if the aspect ratio matches... In fact I don't know how the browser would handle an image that it's asked to resize if the aspect ratios don't match. If you want an image to have a height of 300 and the aspect ratio to match that of a 56 by 75 image its width should be 224.
This seems like the simplest solution but if it doesn't work let me know and then we can start digging into changing and creating variables!

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