If you own image editors such as Paint.NET or Photoshop which support layers, then it's easy to create a custom wallpaper based off of two existing images. First, the main "background" wallpaper needs to be opened. This can be a file of your choice. Afterwards, create a new layer and then open a second wallpaper file. Make the layered image semi-transparent (to your liking).
Don't worry if the image loses it's contrast, this is normal.
Resize the layered image so it's resolution matches the background image's size. If you don't like what you see, try opening other files (either the background or layered overlay) Don't blend more than two images at a time, because this causes the overall image to greatly lose it's contrast, and becomes an eyesore. However, seeing how the background and overlay image are seperate layers, they can be adjusted independently, meaning that one image can be edited without affecting the other image. Adjust one or both images as necessary to your liking, which may include any sharpening, blurring, distortion, color changing, brightness, contrast, or any filters you want to enhance the overall look of the blended image.
To apply the new blended image as a wallpaper, you'll need to save the layered image as a JPEG file. More than likely, your image editor will ask you to flatten your image upon saving, and click "Ok" or "Yes" to proceed. Once it's flattened, than you can't independently adjust the image anymore. It becomes just one image.
This is possible in Microsoft Publisher as well, but you will need to output the image via the PrintScreen command, then paste it into your favorite image editor and save as a JPEG file.
I started wallpaper blending last year using Microsoft Publisher as an experiment; but now I use layered images using Paint.NET because using a real image editor, unlike Publisher has much more control on how to edit each image independently.