I have uploaded some thousands of product with 1 to 5 images each, so a consistent amount of images after all. After the upload I reviewed all the images one by one and they were ok.
Now, one week has elapsed and randomly some products have lost the image associated: on the frontend, I see the placeholder, and on the backend I see a white image with a red cross in the upper left corner. This happens on around one tenth of all products, so I can't see how this can be caused by permission issues at all.
Refreshing the images cache didn't help. The only way to fix the problem is to reload the images of the corrupted products.
Here is the html image code in the frontend:
<img src="MyDomain/media/catalog/product/cache/1/image/512x512/fa01ed8f021de4a5438a05fb5586c842/placeholder/default/logo.png" alt="MyArticleName"/>
And here is the code in the backend:
<img width="100" alt="" src="MyDomain/media/catalog/product/0/7/MyImage.jpg" />
This is a non-existing path (folder 7 is missing).