1. "Who decides..." in general I would say the casino GM or owner or someone of official capacity has the final say on chips. I would also say that most chips at least 80% are mimics of the offical logos that are put together and desgned by graphic artists and garner the casinos napkins, ashtrays, coin cups etc.. As far as LE's the chip making companies employ graphic artists as well who come up with the art then submit that to the casino for approval then to gaming for final approval before they are manufactured in quanity, this is where the notched sample comes from.
2."..submit a design" That's a good question I guess you would have to be in the know shall we say and I would guess that the casino would have to have a need for your design i.e. approaching a certain casino that always puts out a Halloween chip may be a possibility if you are well intentioned. To approach a GM and ask him/her to make a chip around your idea seems like a stretch.
3. "..rewarded finacially" certainly if the image is a copyrighted piece of art or photo legally they would have to pay you for the design long before the chips hit the table, again this is why the casinos leave it up to the manufacture to create the original art to avoid that headache.
These are semi-educated answers hope they help, Eric