imagenconv is a simple command-line front-end for the imagen library.
It doesn't give you access to most of the features of imagen, but is still pretty useful for converting images.
Usage:

Here's an example of loading a PNG image 'input.png', flipping it vertically, and saving it as a BMP file:

You don't have to provide any transformation arguments unless you want to.

...and that's about all there is to it.