Is there an app that would "trick" my phone into thinking that a photo is input from the camera?

Im not sure mate. For what purpose?

 

Ahh lol i see what you mean hmm i duno. Tbh ive never used snapchat so i know what it does but im not sure how it works....
..how close to sending does the pic have to be taken?

Maybe you could just use a file explorer like the My Files app and go into the DOWNLOADS folder and move the file to the CAMERA folder inside the DCIM folder(?)

 

If you're rooted you might be able to pull some shady business with /dev/msm_camera. I googled that to make sure it's the camera device and got this same question from someone else in another language:
foreign language thread elsewhere
No answers there though.

I imagine it may take some more heavy duty hacking skills than merely placing a .jpg in /dev/msm_camera, since the app almost certainly must communicate metadata and control signals with the device rather than merely pulling imagery from the device.

 

Why not do it the low-tec way and take a pic of a pic on your pc lol
(Like half the munters on dating sites probably do)