I recently ran into a similar problem and found that the connector processor was causing the path problems for when adding the base path to the source.
My problem was associated with the modx installation being put into a sub folder (base path: /usr/www/users/userX/projects/modx/ and base url: /projects/modx/). I'm running modx 2.3.3 and gallery 1.7.0.
I found the problem in this file: /projects/modx/core/components/gallery/processors/web/phpthumb.php on line 69
$src = str_replace(basename($basePath), '', $src);
I found that if the website is in a sub folder the basename function only removes the last folder from the path rather than two or more which then also leaves two slashes and in my case part of the base url.
So a thumbnail request like for the image /projects/modx/assets/gallery/1/image.jpg:
The processor then transforms the input source to:
I replaced the above piece of code with the following:
$baseUrl = $modx->getOption('base_url', null, MODX_BASE_URL);
$src = str_replace($baseUrl, '', $src);
This then caused the processor to strip more than one folder from the path including the slash "/".
The resulting input source would them become:
Which is correct.
I'm not sure if the code should be replaced in the gallery project's processor to perhaps to resolve future problems, because even if project is located in /projects/modx/projectA/modxWebsite/ then the base url will be /projects/modx/projectA/modxWebsite/ and it will remove it appropriately.
Hope this helps.