I figured this out! Finally a question I can answer!
All you have to do is upload the big image to the product in the admin just like normal. You probably already have done that. Then you need to add two rows to the configuration table so that you can define the max width and max heigth for displaying the large images.
I use phpadmin, and here is how I did that:
I searched the configuration table for rows where configuration_group_id = 4
4 in my database is the configuration group that is your admin images configuration section. (you might need to check in the configuration_table to make sure it is 4, or you can do your search a different way... like look for SMALL_IMAGE or something)
Then you need to do is create two new rows in the configuration table. I used the SMALL_IMAGE_WIDTH row as my template for creating the 2 new rows.
What I did in php admin was to edit the row that had SMALL_IMAGE_WIDTH
in it... after you do the search you'll see it. Use the little pencil icon to edit it.
Now be careful change the word SMALL to LARGE everywhere it appears in the data for that row. Bake sure the configuration_id is set to blank (a new id will be created when the row is created). There is a little radio button that appears under where you edit the fields, set that radio button to 'insert as a new row' and press go.
Do that again step again to create a LARGE_IMAGE_HEIGHT entry.
(tip: you might also want to mess with the sort order fields so you can get them to display in your admin in the order you want to see them)
NOW you don't have to edit the auto resize at all... Just anywhere in catalog side of the store that you want the large image to appear... change the size and width variables from SMALL_IMAGE_WIDTH and SMALL_IMAGE_HEIGHT to LARGE_IMAGE_WIDTH and LARGE_IMAGE_HEIGHT... Don't do it everywhere or you will replace all your thumbnails with large images... just change it where you want the big ones to appear... I did it in my products_info.php file and my pop-up file.
That's it. No changes to auto-resize at all. Just make sure auto resize is working first.
Kelly