What should I consider when selecting an air compressor for the garage?
You don't really need an "air compressor" in the sense that you don't need something that has a tank and is designed for use with tools.  All you really need is sometimes called a "tire inflator", obviously it is actually an air compressor but it is very different from those listed by Jeff's answer.

I have seen two basic types.

There are 12V versions that are designed to simply plug into the cigarette lighter outlet in your car.  This is nice because you can refill the tires anywhere.  When I had a slow leak in my tire I put this thing in the trunk and drove to work, then before leaving work I would top off the tire again.  It worked fine for the day or two until I could get to the shop for a new tire.  Usually these are pretty cheap.

The other version is designed to plug into a household receptacle. These seem to be a little more expensive than the 12V versions.

Sometimes you can find these types of tire inflators with standalone battery jumpers attached so you can also use it to jump your car.  I would skip that feature unless you think you would actually use it.