Comparing images and producing co-ordinates

I'm looking for free software that will allow me to take 2 images from
a web cam and recognise the differences between them.
For example in one image an LED would be off and in the next it will
be illuminated. Is there any software that will be able to recognise
such a difference - and produce coordinates to locate the item (in
this case the LED)?

I'm new to this kind of thing so it needs to be pretty simple!
I've been told that certain image formats are better for this kind of
processing. I'm not sure what they are and searching the internet
hasn't produced any results.

Hope somebody can help! Thanks in advance.

James