Hello William and good day to you too.

Firstly ensure that the problem occurs only for images sitemap precaching this ensure that the problem is server side only, if so you have to investigate in the server error log to find more informations about the error.

It's a generic server error that could depend on multiple factors, for example PHP or server settings.
Indeed the images sitemap differs from other ones because a network crawler is used.
Just a tip: to test if the crawler is working or not on your server you can use the crawler test button as visible in the screenshot. That must render your website home page, if not it means that it's not allowed by server settings, in such case you have to ask to the hosting provider.

Regards

John