Overlay image is a static semi-transparent image which is inserted between your wallpaper image and calendar. Put your company logo in there and it will be visible all the time in your desktop or use it to enhance the readability of desktop icons.

Create your own overlay images - here's how:

Method 1:

Design a picture (can be any size - it will be stretched to fit your desktop)
If your image has transparency information (alpha-channel), save it using image format that supports transparency (for example as a .PNG-image) and select it as your overlay image.

Method 2:

You can also save your image without transparency information and in this case the intensity of each pixel in image determines it's transparency. The highest value of any color components (Red, Green or Blue) will determine the transparency level of each pixel. Black is totally transparent. Example: if color value of the pixel is (in hex) $a0e830 the transparency level will be the highest color component which in this pixel is $e8 and this makes it almost opaque. Another example: color value $e80000 would give the same transparency value ($e8) as in first example.