A rebus puzzle has one or more squares in the grid containing a multi-letter word that represent a particular picture. For example a puzzle might contain the word crossroads where cross is all in one square. Optionally the square can also represented by a picture, e.g. of a cross.
To make a rebus puzzle manually use Square Properties to set the multiple letters for a square and to set the square's background picture. Check the Rebus option box so that the picture is shown in the solution but not in the puzzle. When you edit the grid you will see both the multi-letter word and the background picture. You can also use the Ctrl+I shortcut to quickly insert multiple letters in a square (the same shortcut can be used by solvers using the solving applet).
A sample puzzle is provided called Rebus demo that you can look at as an example.
The Pro Grid Filler also supports setting rebus words when filling, automatically using multiple-letters in each square where they appear in the word list. See the Pro Filler setting tabs.