High - resolution images..

A lot of talk is generated about the resolution of images from MER,Viking,
Sojourner, Hubble, Mars Express....
The main thing seems to be comparison which images are of higher resolution
than some others,
even comparing Spirit images of Mars with 'images I make with my new cool
digital camera'.

Now, the big question mark appeared above my head when I realized I don't
really know what
these people are talking about?

What methods are they using to measure the 'resolution' of one image, say an
image of Martian lanscape?
What units is it expressed in?

The only meaningful definition of image resolution I know of is spatial
resolution, or
the minimum angle between two points resolved on the picture, in radians.

But it seems like the popular 'resolutions' are defined either with
'meters/pixel' which is easy with a microscope, or 'megapixels' which is
again easy with Photoshop.
?
JD