This is a plugin for Paint.NET that replaces all segments of one image with equally-sized segments of another image.
Store Image to Replace: Stores the first image, which should be replaced.
Store Replacing Image: Stores the second image, which must be the same size as the first and will replace all copies of it.
Tolerance: How much each pixel is allowed to deviate from the image to replace. Used mostly on lossy-formatted images.
Reset: Discards the stored images.
Replace: Replaces all copies of the first image with the second. It may take awhile.