All photo's previews are protected by a watermark.
Please go to Admin Panel -> Settings -> Watermark
You should upload a transparent *.png file with your logo or an inscription there. Also you may define a position of the watermark on an image.
Question: I uploaded a new watermark but I cannot find any changes on the site. Why?
The photos with watermarks are generated one time during uploading. The new watermark will affect new publications only. You can regenerate the thumbs here: Admin panel -> Catalog -> select action. Attention! It can overload your server.
Question: I uploaded a watermark but it shows at the top of the thumb.
The watermark width must be smaller than vertical thumb's width.