I had a similar problem when trying to use Gallery on my local PC (WAMP with Apache 2.2.21 on Windows7).
Debug in Firefox gives me something like 'image corrupt or truncated....).
First thing I found : a problem with Apache on Local that doesn't allow encoded slashes (%2F instead of '/' in the URL), that causes me problems with all the thumbnails (this is due to the way phpthumb generates names for the thumbs as far I understand). I added the directive 'AllowEncodedSlashes On' to my Apache conf, that solves the problem for things like ImaplusTV but not for Gallery...
I think it's an environement dependant bug but very problematic to develop on local when you want to use Gallery...
The only workaround I found at this time is to use an older version of the phptumb.class provided with Gallery, but it's not ideal of course ... see my olders posts on this subject (
https://forums.modx.com/thread/74834/apache-on-wamp-and-url-s-with-and-gallery-maybe-others#dis-post-414967, https://forums.modx.com/thread/74792/gallery-1-4-possible-bug-in-gallery-processors-web-phpthumb-php#dis-post-414870) I posted also an issue on github...
Cheers