I think your Imager module got installed without some of the desired (but not required) libraries installed first. I would advise you to check out what library you're missing, but I'm virtually sure it's the IJG (Independent JPEG Group)'s widely used "JPEG Source". I guess the workflow is: Uninstall Imager, install JPEGSRC, install Imager. You might be missing more libraries, check those out first, too.

I can see there's a list of external libraries in the README file. And indeed, it uses the IJG's library.