no difference

Bitmap checkpoint compare the image ie pixel to pixel 
comparison is done but whereas in Image checkpoint the 
properties of the image is also verified along with the 
image, like alt tag of the image and anyother property of 
the image and the Image checkpoint is inerted only for the 
webapplication ie..when u insert the standard checkpoint 
for the webbased application for the image then you get 
this checkpoint

image checkpoint is used to capture the desire location
bitmap checkpoint will capture whole picture

Image check point is used to check the GUI properties of an 
image.
Bitmap Check point is used to check complete or part of 
BITMAP i.e picture to picture.