By request, I'm posting a quick summary of how collections will work. I think I did this already, but the description must have been lacking much.
A collection will be its own "type", and will be constructed differently.
It will have its own category type, where images in that category are assigned to the associated collection.
Prices of collections will be determined as the sum of all image prices, most likely discounted by you by a given amount.
Collections will have a special graphic you make - completely controlled by you - a description, as well as the listing of images below it.
Upon purchase, all images will be assigned to the download area.
The collection plugin will be sold as an upgrade.
There is the possibility that you will be able to download a zip of images to save the customer work, but ZIP features do not work on all hosts as I've learned. We will see how we go about that.
Collections were my main income on clipartillustration.com before the overhaul.