Here you can select the image you wish to overlay over the current map window.

Using the pull-down menu the prior loaded raster images can be selected.
If the pull-down menu is empty you need to load the required image first using 'File > Open picture'

Checkbox-'White is transparent'

When this box is checked the color white is treated as transparent.


You can Copy, Add, Multiply, or bitwise overlay the selected source. See 'Merge pictures...'