Are you selling your own manufactured goods or reselling someone elses? If you're reselling someone elses, don't they have images of their products already to use? I think it's rare for a reseller to take all their own product images, but if you do because you feel it gives you an edge, I would use your own logo prominently rather than as a watermark. Maybe you have an artistic presentation of goods that needs proprietary protection - that is the only case I can see for potential watermarking.
And, if you're selling your own goods, do you also sell them to others for resale? I frequently need product photos from manufacturers for promotion by authorized sellers of their brands.
Different companies have different ways they let marketing depts retrieve images, and from my side, the easier it is, the better. I've never seen anyone put watermarks on their images, but some create their sites so you have to go through a lengthy registration process and wait to hear back - huge pain in the kisser for me as I have to go to the client, they have to find the account, get it back to me, I enter it, oops, website says not that string of numbers, go back... etc. Time suck warp.
Some companies make their images non-downloadable (though there are some workarounds and screen shots). The ones I like best - just let me have the images, no mess, no fuss. I am after all trying to sell their products.
Then, I brand a corner with the resellers logo, but not as a watermark; rather it's a notice that yes, this company carries this merchandise. And for SEO, if the reseller branded image of the original product comes up in image search (which they do sometimes as I always fully tag out my images), then all the better. If you're selling your own goods with no authorized resellers, I think I would brand a corner also - no watermark, just a logo, but only if for some reason your logo is not already on the products.
