this site has the effect I am talking about It's a watercolor illustration image and has a white dotted pattern on its ending and fewer dots on the center I want to know how to make this effect.
Insert a new layer. Paint to it white with a soft brush. Let the brush have reduced opacity, say max. 30% and blending mode = Dissolve.
I said the brush! The new layer should have 100% opacity and blending mode = Normal.
Apply to the new layer Gaussian Blur 1px or less.
The upper dot cloud has got no blur, it's only a couple of brush strokes as described above. There are random white pixels here and there as blending mode Dissolve makes. The bottom side dot cloud has got 0,5px Gaussian blur after painting.
In both cases the brush opacity was 25%. The image (when painted with white) was a 600 px wide randomly selected snippet of a news photo.
Make some tests. The visual result depends much on the pixels per inch of the image. If you have a high resolution image below it's well possible that you must scale the resulted dissolve pattern bigger to make its apparent density and grain size to fit the image below. Do not insert blur before you do the scaling. Scale by using the nearest neighbour resampling mode to keep the dots sharp. Other resampling modes make the dots blurry, but that can also be a good effect.