Sorry, but I can't help you on the first one, I could give you some links to known free/low-cost shopping cart programs/scripts, but haven't used them so I don't feel comfortable promoting them since I don't know the quality of them.
As for your second question, to get great, high quality, high-resolution photos, you can go to
http://www.sxc.hu/ They have free stock photos, that are also royalty-free.
And for clip art, you can go to
http://office.microsoft.com/clipart/default.aspx - If you have Microsoft Word or any program from their office suite, you can go and d/l the clip art from there. Or, if you just want basic clip art, a simple Google search will give you plenty of links to royalty-free, and monetary-free clip art.
David