Cannot add images to the store

I want to use images as products. When I edit some image and change its product type from "Not a product" to the "Tangible" and pressed "Add to store" button nothing happens. No price is show only page is refreshed and "Tangible" is selected. When I press "Submit" button I get error "Please enter a numeric value for stock."

It works when I create new image, then I can enter price etc. after I add image to the store, but I cannot add existing images to the store.

What may be the problem?

Thx.