I need help identifying my derailleur
As @jimirings said its hard to tell from the photo. 

To identify which derailleur you have there are a few options:


If this is the original part on the bike use a site such as bikepedia to determine what came on the bike as new.
Look at the rear shifter pod, if this is SRAM it is more than likely it is a SRAM derailleur, if it's Shimano its more than likely SHimano, they are not generally compatible.
Count the sprockets on the rear cassette, this is a good starting piece of information to help working out what it is.
Find any words on the cage, something is written there, google this.
Find any other writing on the derailleur and google this.


Derailleur cage length is determined by largest chain ring and cassette cog. Once you know what derailleur will work you can calculate what cage length will suit.

From what I can see from here, I'm guessing you're probably running something similar to this derailleur