I usually resize to something like 1000-2000px wide with just enough compression to not be noticeable while still keeping filesizes reasonable. The problem is when people upload massive photos without any optimization and it bogs down the page. Unless it's photographed well it's just a waste (if it's photographed well though I don't mind
).
The downside to not watermarking is you get A-holes that use your work to promote their own BS, like I saw ripster do to some poor schmuck.
"ripster55 0 points 25 days ago
If you don't mind I did a slight edit for welcoming new members to the Mechanical keyboard Club!"
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I've seen Ripster remove copyright on other things he's posted, so it's not like that necessarily helps. Adding watermarks though is a fairly extreme method of signifying the original author. Most good photos of things online have no watermarks and they look better for it.
Unless someone is selling something, or expect someone would copy your photos to sell something, watermarks seem a little overboard imo. A small solid name in the corner would suffice otherwise if one wants to label it. The worst is when you go to flickr or some such site and they've put a gigantic watermark over a rather average photo. In my mind the better or more well known the photographer or artist the less likely they'll use watermarking, while the opposite seems to be true.