1st step would be to look through the guide's index
CS5 User Guides - online and PDF
I think what you are looking for is usually called green screen (since the traditional fade out background is green)
I'm not sure that keying is the right approach (and if it were, I would use green or blue, not red).
Read up on "motion tracking" (but I prefer After Effects for this purpose):
Also scaling (you've got to gety our replacement images the right size and in the right location):
Personally I think it would be less work to just reshoot with the actual pictures on the wall.
AEFX and or Mocha is the only way to do this (apart from hi end 3D applications.)
Then it will depend on how well you shot it.