Click the Image tag and type in or browse to the name of the image file you want to place on top of your image. The image can be any supported type i.e. BMP, TIFF, PNG, GIF or JPEG.
Make sure image captioning is switched on and the button in the top right shows: 
Set your required location, opacity and margins.
Transparency
Transparency (if required) is supported in several ways:
- Transparent GIF or PNG. If you load in a transparent GIF or PNG file Image Optimizer will make the transparent color transparent automatically.
- Set transparent color. You can manually set a transparent color by selecting the “Trans” check box and clicking the “Trans” button to make all pixels in the image that match this color transparent. This may be useful for 24 bit BMP, PNG or TIFF images.
- Alpha channel. If you want sophisticated transparency such as antialiasing and feathering use a TIFF or PNG with an alpha channel. An alpha channel lets you control the transparency of each pixel. You can use most good paint packages to make these files. Normally these files will use “pre-multiplied alpha” and you should check the box. If you can make images without a pre-multiplied alpha channel you will get better results and you should un-check the box.