You can create your own thumbnails in Photoshop and replace the ones generated by svManager using the same file names. Some servers may give problems with overwriting files that have been created by a script but you probably won't hit that problem and there are usually ways around it if you do.
There is a drawback but you might be happy to live with it. SvManager will continue to display square thumbnails in the images and sort pages so they will look a bit squashed but everything should work ok (I think!).
You'll probably want to change a setting in the svmanager/includes/constants.php file so your carefully crafted thumbnails are not overwritten if you hit the Rebuild button.
If you are using the automatic index page then you can change the size of the thumbnails there with two more settings:
The thumbnail images for the index page are cached and the cache is updated every time the gallery settings are saved so if you care about the index page and you've been creating your own thumbs then hit one of the Update buttons – say the one on the Customize page – to update the cache.
Doing it manually like this would probably be a good first step. If you are confident with php and want to experiment with the code then the area to look at is the createThumb method of the Gallery class that lives inside svmanager/classes/gallery.php. Usual warnings on mods and hacks apply – you may not get much support and you might have to do it again when the next svManager upgrade is released.
Hope this works for you – let us know how you get on.
SimpleViewer Support Team.