May be analysis of satellite images - a lot of sources of images, a lot of companies, a lot of literature.

Medical Imaging and CAD based diagnosis; Remote sensing and Weather forecasting 

I would like to recommend you medical image. 

Surveillance imaging would be a favorite subject among researcher. 

on the other hand i would suggest the face recognition image process this the challenging factor now a days  

I would recommend  Synthetic aperture radar (SAR) images . satellite image can be work with various methods in different applications.

I'd say the "best" area is the one that stimulates your interest. Two other areas not mentioned so far: consumer applications of augmented reality on mobile devices and industrial vision used for inspection of many manufacturing processes.

 Image Analysis (Synthetic Aperture Radar)