For the past several years, we have been adding a watermark to images we have purchased from 3rd party photo vendors.

The reasons for why we do this are many and varied:
Remember the adage "A picture is worth a thousand words"? It refers to the idea that a complex idea can be conveyed with just a single still image. It also aptly characterizes one of the main goals of visualization, namely making it possible to absorb large amounts of data quickly.
Because you only have a second or two to capture the attention of a new visitor, using imagery that is related to your web content could mean the difference between a visitor hitting their back button or staying around to see what your site has to offer.
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