Thumbnail issues when using a mobile theme (thumbnails on 2 themes?)

I’ve been trying to figure this out for the last few days. I currently have my site setup with both a desktop and mobile theme. The mobile theme is loaded with one of those plugins that detects the device and then loads the theme. I’m using Bonfire’s Vibrant mobile theme. The site is: https://www.factionhub.net/

Here’s my problem. The desktop site’s thumbnails/featured images load correctly, but on the mobile theme they look fuzzy. I’ve only found two “fixes” for this.

I either have to activate the mobile theme and use the regenerate thumbnails plugin to make it work–but doing that messes up the thumbnails on the desktop site when I switch back to that theme.

The second thing I did was go into Settings > Media and adjusted the thumbnail size to 550×480. That fixed the mobile theme issues, but–again–it messed up the desktop site as all the images were no longer cropped to the correct dimensions, so the front page looked like a mess with a bunch of different sized thumbnails.

I then unchecked the “Crop thumbnail to exact dimensions (normally thumbnails are proportional)” box. It fixed the desktop site, but once again messed up the mobile ones–they look fuzzy again.

Any ideas on how to fix this? Is it possible for me to use thumbnails on these two separate themes? Thanks!