Thanks to Sergey for reminding me of the special nature of the MDIParentForm !
Once you have a valid reference to the MdiClient Form ... which is not visible at design-time ... as shown in this (Jan. 3, 2014) QA answer: [
^]:
1. You can draw lines on it using the Paint EventHandler, as shown by OriginalGriff's code: [
^].
2. or, set its BackgroundImage, as I suggested in: [
^].
The MdiClient is a Form, and no special API calls are needed to use its Paint Event, or set its BackgroundImage.
However, please keep in mind the cautions I stated in the cited answer to the question on defining a Click EventHandler for an MdiParent Form: that using the MdiClient is a kind of hack. I would avoid using it unless absolutely necessary.