Use photo file instead front camera image Instead of using the real camera, I would like to simulate the camera input as a static image file. 

I need to use it in one program, where I can't use 3rd party "fake camera apps" because program asks me only "Do I permit this application to take photos and record videos" and after "Allow" my front camera turns on. I can't even choose the 3rd party fake camera app. 

Any suggestions on how to deal with that?
As a developer, I don't think that's possible without changing the program you are trying to use. 

If that specific program(I suppose it's actually an application) was built to use only the images taken by the integrated front camera, you have to first "teach" it who to use images from other sources. But that's the job of the program developers. You can probably contact them and request this new feature.