Adding a watermark at one corner of the image may not always serve the purpose. Look at this photo.
The trick is to add a very light watermark which is tiled across the photo. The tiles should be spaced so that there are not too many watermarks and should have a high level of transparency. Just enough to make the underlying photo visible but at the same time making the task of removing the watermark almost impossible.
It is better to rotate the watermark rather than have a straight watermark because that way there is more gap between the watermarks and it also looks pleasing as rotated tiles make a pattern which looks better than simple tiles.
Tiling watermarks with uMark is very simple. You can either tile a watermark horizontally, vertically or both which will cover the entire image.
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