How to compare 2 images from command line

[FreeBSD]

Hi everyone,

I'm trying to determine if 2 jpeg images are identical. The images are 
screenshots taken with scrot at different times. The point is to know if 
the display is working correctly. I tried to use 'diff' but without 
success, probably because of the metadata included in the image. I also 
tried the 'compare' command from imagemagick, but it produce an image 
containing the difference between the 2 images instead of telling me if 
both images are identical.

So, my question is what are you using to determine if 2 images are 
identical?

I'm using FreeBSD 7.0 and I need to be able to script this comparaison 
for an integration in Nagios.

Thank you,

Martin