Well I'd say that you can do what you want really... use a 1 day membership as the basis for each picture/drawing.

Set each picture up as a separate 'sub account' so it leads to a specific download page for that drawing

Each sub account can be whatever price point you want. This way the 'buyer' can download without having access to the other downloads... course its a bit of bitch to set up if you have 100's of pics.

Another option is to look into a shopping cart / oscommerce type site but seriously no reason why you can't do separate 'sub accounts'. It's what I do for my books and works like a charm... one pswd file, one script, but numerous sites all pulling from the one... random generated sub directories so they can't guess an order and gain access... plus I use a d/l management program in addition to record the ip and insure no one is d/l more than one copy of the file.

just my pennies worth
hth
Ian