Hi Duncan,

trying to explain how images sitemap works probably will help you to understand better.
It acts as a crawler that scan pages on your site finding images embed in HTML code. It's the very same system that a google bot uses. For this reason the process may require several minutes if you have a lot of links and if server timeout is low you receive script timeout.

Images sitemap has filters that let you specify what images have to be excluded/included, the categories exclusion is another type of filter available in th 'content' data source and exclude categories in all types of sitemaps.

Keep in mind that trashed or not trashed files should not affect crawler because it grabs images on your pages and if the path have no exclusions the image will be added to sitemap.

Finally we have started developing a new system to generate large image sitemap avoiding server timeouts, thanks to an incremental process step by step. This feature will be available in next version 2.3.

Thanks and best regards

John