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.
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