Tastes vary, but I've never seen an image with a watermark that wouldn't have been better without it.
The disadvantage of using a watermark is obvious. It introduces a distracting visual element that doesn't belong in an image. The effect of a watermark on an image ranges from mildly distracting at best, to ruinous at worst. When I see a photograph with a watermark, the watermark is almost invariably the first thing I look at. If it's large and obtrusive, it's also usually the last.
Besides Moshe Zusman's tips for online marketing, you should consider other ways to ensure that you—and others—will be able to track your photos online. At the same time, you must pay heed to copyrighting your photos. It's not all about watermarking either. Here are some tips on how I do it in Lightroom 3.
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