How can I vent a bathtub trap when the drain is routed under the tub?
Why do you think the bathtub is not currently vented? 

Not all traps need to be individually vented so you're not always going to see a vent if you don't know what to look for. It's a little hard to tell from your photos, but I think your bathtub is wet vented through the kitchen sink:



Please correct me if I've got the arraignment of your pipes wrong.