I've built a small scene by following a tutorial in “magic of houdini” book. I can't find where houdini saves the 2D image file that I rendered and saved. This is probably a stupid question, but I'm new to Houdini and find it quite difficult.

By default, I think it gets saved to the directory that Houdini was started from.

the easiest way to find it may be to attempt to save a new image. Check the file browser to figure out which directory that you are saving to by default.

This directory can be different pending the operating system.

On windows, chances are, it's something similar to CDocuments and Settings/stucker/