_d_improd_ directory in uploads breaking site images

Does anyone have an idea where this folder could be coming from or be caused by?

This folder is not created in a plain install of WordPress but it could be created by the Image Pro – Image resizing and media management done right plugin (no longer maintained?) that handles image resizing among other things.

I tracked down this part of the plugin:

const GENERATED_FILE = '_f_improf_';
const GENERATED_DIR  = '_d_improd_';

that seems to match the folder you described.