How can I take photos of a model in a jungle or tropical setting, in my own "backyard"? I want the effect to look real, not like a fake backdrop.
Find a stock background image that has the look you're after.
Shoot your model on a plain background. Make sure the direction and color of the light matches as closely as possible
extract the model from the background. Using a dark green background might make the masking easier.
shoot in a park or backyard with lots of greenery, using a shallow DOF to hide the fact it isn't a real jungle (or as Imre suggested, shoot at night)
hire or borrow some palms or ferns to use in the foreground. I think this would "sell" the idea it's a jungle, with the background too fuzzy to give away it's not.
or a long shot, but if you have a local zoo, they may have some areas you could use that have ferns, bamboo or other "jungle" looking greenery.
Concentrate on the Foreground
Good foreground objects will sell the effect.
For example: if your background (be it canvas or a stock image) contains palm trees, then some palm trunks or even real palms in the foreground will build an connection between the fake and the real.
The larger / brighter / more in focus the foreground elements are, the more they will draw the eye away from the background. Also, of course, the larger they are, the less actual background will be visible.
Can you find a location where you can shoot the model in some jungle-type foliage with a green-screen behind?
Imagine a shot which is looking out at the model from behind 2 big jungle-plant-type leaves. The background can be pretty much anything with trees in, and the effect of being in the jungle would be sold. An appropriate outfit for the model will help sell it too.
You might get a jungle shot quite cheap if a night shot is okay - then you won't have to make all background look like jungle. It's enough if you have a palm tree or two glimpsed by lighting giving hint that it's a jungle, and the rest of the background can be left dark. Some florists provide big exotic plants for rental. Or they might have some big leaves which you could set up on a lightstand.