Under Windows and Mac OS X
Select in the plan the items you want in the PNG file.
Choose Edit > Copy out of the menu.
Launch Paint under Windows, and Preview under Mac OS X.
Choose Edit > Paste, and save the image to PNG.

As the menu item Edit > Copy may not work under Linux, print to PDF only the plan (choose the right option in Page setup dialog), and copy the image in the PDF to a drawing program.