You could extract the xml code of the toast window from MsgPlusLiveRes.dll and edit it to contain a picture (or use the one that has the picture container in it already)
The dll is located in de Messenger Plus! Live installation folder. You can extract the resources using e.g.
ResHacker.
Using this method, you'll have to show and hide the window yourself (through scripting), so you can't use MsgPlus.DisplayToast().