Excuse my ignorance, I don’t understand the concept of ‘advertising bag’. Is this something like a plastic bag that people use to collect the leaves of their property? I saw them sitting on lawns, usually with a halloween print. Or is this a tarp-like material that sits on a metallic (wire) frame, but it is baggy – as opposed to a solid plastic plate?

Yes, bags are made of tarpaulin like material. We use thick white (or yellow) bags that have black layer inside so you can’t see through a bag. Such a bag is very thick and stretched on U-shape metal frame it looks like a solid surface sign.
The most of images on our web site are actual photographs. You can try the following page to see a sign under a better angle.
These bag signs are more popular nowadays than traditional coroplast (corrugated plastic) signs because they are cheaper and stronger.