Using AutoThumb, you can generate thumbnails for all your images in DOCman. Just set some parameters like dimensions, image format, quality,... and publish the mambot / plugin. When browsing the frontend, thumbnails will be generated for all suitable documents.

For DOCman 1.5, the extension was renamed to "DOCman Thumbs" and is now part of the DOCman Productivity Pack, available at http://joomladocman.com .