Do you mind testing a new version of this RIQ plugin for me? I have a new version on Github, https://github.com/mbijon/WP-Resized-Image-Quality/archive/invalid_header_wporg.zip
* Deactivate & then delete the current plugin
* Download that ZIP and put the 'WP-Resized-Image-Quality-invalid_header_wporg' folder into your server's /wp-content/plugins/ folder
* Rename that folder to the same as the production plugin: "wp-resized-image-quality"
* Activate this new version of RIQ from the Plugin screen in wp-admin
Anything to report, good or bad?
1. - This fixes the ability to set a compression level of '0'
2. - I wasn't able to reproduce the "Invalid Header" error. That could be a conflict between my plugin and another one ... or we could get lucky & see it fixed by this version.