well, your notion
...if the user could access their download log from their control panel...
would be a very good extension of such a mod!
you hit the crucial point of my idea (ooh, it´s not my idea, really not...
)
but that:
this type of mod could probably be handled better by a commercial shopping cart...
is absolutely not necessary. far from it!
the usual way in selling pictures is to evaluate a price concerning the purpose of use of the downloaded image. there are many many factors who affect the value.
so you have to wait for a message like: "i have printed the picture in this ... magazin and that ... size with that ... print run" and so on...
but it´s going too far to explain the whole thing.
all you need is something as a logfile to document the download sessions (with filenames etc.).
i think the easiest way would be to analyze a "normal" serverlogfile and extract the special things to import in a field in 4image database??
so, what means this for the desired mod?
this is much simpler to code it, isn´t it? ...but not for me, i´m only a photographer and not a software engineer...