Revo 2.3
I see there are many threads about this problem but few resolutions. I'm having the same issue locally (MAMP Pro) as well as remotely. When I was running MODX 2.2.x thumbs were fine.
I'm not seeing any thumbnail previews in the media browser or anywhere else on the back-end (MIGX etc.). I've tried all the System Setting suggestions for phpThumb with no luck. I use A2 hosting and they use the Image Magick binaries on their shared servers but apparently not the PHP extension
http://www.a2hosting.com/kb/developer-corner/linux/using-imagemagick. Is that the problem or should I be looking at MODX?