I just tried to scan and generate an image sitemap for ~100 images on a test web site. In "analyze website" I deleted files that I don't need and entered titles/descriptions. When I generate the image sitemap there's just 1 image in it?! If I generate the text sitemap, all the images are correctly listed. I tried to check/change some options, but nothing helped. What can I do to try to detect/correct the problem?

Images have to be associated to a page when creating image sitemaps for Google.

Hence if you deleted page URLs, A1 Sitemap Generator can not generate a correct image sitemap.

See this help page for more information:
http://www.microsystools.com/products/sitemap-generator/help/image-xml-sitemaps/