Another strange thing: After importing products, some of them don't have their images assigned: They have a file id (fid) of 0 in the content_field_image_cache table and entries for the images are missing in the files table. I fixed this by inserting a new row for each file in the files table and updating the fid in the content_field_image_cache table. (I also updated the sequences table.) Now the images do show up in the product list, but they are missing on the product detail page. The product_image div is not even rendered in the html.

Does someone know what is influencing whether or not images appear on the detail page? Thanks for any insight.

Edit: It was a cache issue - I cleared the cache_content table and now the images appear on the product detail page...