I am building a medium format folding camera database for readers who ask the question "which MF folder should I get?". The result will be a report that will list the camera names and then sort them by their features (coated/uncoated lens, coupled/uncoupled rangefinder, etc.). I have the opportunity to add a thumbnail image of each camera in the listing. This will help readers differentiate between cameras with the same name but different features (I have seen three very different versions of the Franka Solida II for example). I would like to copy images that I find on other websites to include in my database. I would then link each image back to its original page. What are the ethics involved here? Do I have to get permission from each image owner before I use it? I am not making any money from this and I will probably generate traffic for their site through the links.