Hi Justin
Hope you are well, not spoken in a while! Just for future reference there are very nice mobile responsive templates for OpenCart at themeforest.net
1) Your banner images are saved as .jpg, .png, .gif - resave all these as .jpg (Progressive). Also Photoshop is crap at saving in terms of file size so get yourself a free copy of Faststone image ressizer at
www.faststone.org/FSResizerDetail.htm
2) Your template was designed to be max-width : 1000px with 80px total padding thus max-banner image size should be 920px wide for best looking results.
3) Turn on SEO re-write (and edit .htaccess file to correspond). Plenty of documentation available to do this.
4) There are a few typo / grammar errors on the terms and conditions page.
Selling for local companies / artists
There are a couple of ways you could approach this
Option 1
Create a master category Artists and add a sub-category to this for each artist
Option 2
OpenCart supports multi-store setup (so potentially each artist could have their own independent store) such as artist1.yourdomain.com artist2.yourdomain.com
However I don't use OpenCart (100% Magento sorry) but the only issue that springs to mind is who is taking the payment / who is providing the terms of business, who is liable, etc. ???
I personally think you would be better going with a whole new site with categories and all the artists products combined with a new company formed to run and process the payments which in turn are passed onto the artists minus your commission (so in effect a mini-ebay).