This depends on the nature of the blended (overlay) image and the technique you use to blend the 2 images. If the image has what appears to be a pure background (such as fully white), but it's saved using lossy compression like JPEG, compression artifacts could
cause the white areas at any edge to become non-pure white, such as shades of light gray. If you use a mixing technique that relies on the background being a pure color, these edge areas could come out as 'blocky' pixels.
Another possibility for such pixels arises if you use a transparency mask like an Alpha channel. If the mask has abrupt jumps from transparent to opaque, the blended pixels at these jumps could be a bit noisy. One way to fix that is to smooth the blending mask.
This is explained in this forum post I made on my company's forums:
If neither of these ideas explain what causes your problem, post more information about how you're mixing the overlay with the main image, and if possible attach both images or post a link to download them.
Edited byAmin DodinThursday, April 20, 2017 11:18 PM