Technical comparison: MBP 15" mid-2010 or MBP 13" 2012
Let me provide you with my opinion:


The CPU can not be changed (well it could but at about $1000)
The slower USB is normally not a issue unless you are high user of external disks, etc. (and even then it is unlikely to make a noticeable difference)
Buying an external screen will always be an option regardless of model (only budget question).
Portability? Do you have to travel a lot?


Since your main use is to edit pictures, how about a graphics quality comparison?




  MacBook Pro (15-inch, Mid 2010)
  
  Graphics and video support
  
  
  NVIDIA GeForce GT 330M graphics processor with 256MB of GDDR3 memory on 2.4GHz and 2.53GHz configurations; or 512MB of GDDR3 memory on 2.66GHz configuration
  Intel HD Graphics with 256MB of DDR3 SDRAM shared with main memory
  Automatic graphics switching
  Dual display and video mirroring: Simultaneously supports full native resolution on the built-in display and up to 2560 by 1600 pixels on an external display, both at millions of colors
  


Source: http://support.apple.com/kb/SP582

The Cons:
The MacBook Pro 15″ Mid 2010 had an 'intermittent black screen or loss of video' problem caused by the graphics card. The program has since ended.




  MacBook Pro (13-inch, Mid 2012)
  
  Graphics and video support
  
  
  Intel HD Graphics 4000
  Dual display and video mirroring: Simultaneously supports full native resolution on the built-in display and up to 2560 by 1600 pixels on an external display, both at millions of colors
  


Source: http://support.apple.com/kb/SP649

Further details of the 13″ at NotebookCheck.net