Thank you for the replies.
I don't know much about the .img files. There is no documentation that accompanies them or instructions on how to use them on the S.F. Bay Area Midpeninsula Regional Open Space Preserve Web site.
To answer Seldom's question, no they do not turn pink with a yellow line when I select them in Mapsource. The maps appear to have been correctly compiled, though. They display the park names, regions, topography, elevations, trails, and even parking waypoints.
I will try the SendMap20 tool as Boyd suggests. I didn't realize these containers include instructions pointing to specific tools needed to download the data to GPS devices. I'm not very familiar with the files contained in these images. I tried to mount the images on my desktop but was unsuccessful, so I don't really know what is inside them.