There are two ways to set a image. For example the image attribute is named image.

First way is: image="path/filename".
You know the complete path and the filename (inclusive the extension) to the image and put it directly to the image attribute.
If you set image="" the image is marked as not set.

Second way is to set the path and the filename separate: image.path="path" image.name="filename".
If you set image.path="" the image path is marked as not set.
If you set image.name="" the image name is marked as not set.

Note on transparency of images: You should only use images without alpha channel. Because currently we cannot handle image-based transparency. But if you want to have a transparent image you can use the alpha value of the correspondent color. For example you can load the bgimage="<filename>" of a window or a widget and set the bgcolor="#00000080". The alpha value 80 hex sets the half opacity of the bgimage.