Adding the stamp in preview

Maybe some more detail will help…

Open the image of the stamp in Preview then select Copy from the Edit Menu or press Command+C.

Then open the image you want to paste the stamp on and select Paste from the Edit menu or press Command+V. Once pasted, you can move and resize the stamp where you would like it located.

Add a company stamp to what, an image or a pdf?

If you have a compatible graphic of the stamp you can copy it to the clipboard and paste it onto the image.