If I were traveling with frozen pizza, I'd probably just stack them on top of eachother, surround it with ice packs and styrofoam and put in in a box to check. ORD-DEN isnt that far and even if the pizza melted, it's not that big of a deal. I'm picturing large pizzas, if they are like those small personal size pizzas, maybe you can get away with it in your carry-on but after adding the ice, there's not much room for anything else. If you do check, obviously, its better to do it in the winter.

I travel with frozen foods quite often. (ie. once a month or so).

When it is a frozen liquid (broth/soup) I pack it in a box the same way frozen food is shipped. Styrofoam with ice packs. I check these.

Otherwise when its meats, meat buns, stews, etc, I just use "blue" ice packs. I usually just put it in my carry-on. I had one occasion where I checked the bag and UA screwed up and it delayed 24hrs.
I'd allow an extra 5-10min going thru security as they often want to take a quick peek to make sure its actually ice packs and they are solid.

My longest travel time (with connections) is about 12hrs. If I do check my bag and its a longer flight, I usually try not to do it in the summer.
It is also important to make sure your food is completely frozen and judge by the shape of the food. If its frozen in a large block, it is harder to melt than something really thin (more surface area).