Images sitemap generation is accomplished using a crawler that scan your site looking for images to include in your sitemap.
This operation when you have hundreds of links may require several minutes to complete.
In the configuration of JSitemap you will find a setting to limit number of requests used to generate images sitemap. Reducing this value can decrease the total time needed to complete sitemap generation and avoid server timeout. By default the parameter is set to scan 50 links. You can progressively increase this value if you have more links, trying to generate the sitemap and ensuring that the process completes without server error and timeouts.
Of course this may have the side effect to not scan fully your site and so skipping some links and images. In this case you can better use the advanced AJAX generation and precaching features added in the version 2.3 of JSitemap to have a full images sitemap generated step by step.