Fix some rough edges when handling large pictures or full sheets

- Choose Picture dialog now scales these pictures down to fit in the available space, thus avoiding the ugly overlap issue.
- Choose Picture dialog now supports full sheets, scaling them down in a similar manner
- Due to the above, the Display Picture node now offers a Choose button
- The possibility of large (72x72) dialog pictures is now properly documented

doc/editor/Graphics.html

@@ -134,6 +134,14 @@ consecutive custom sheet slots (pushed into the upper left corner).</p>
 put 4 in the Picture Type field. Add 1000 to the slot the left
 half is in, and put that number in the Pict spot.</p>
 
+<p><b>Large Dialog Picture -</b> This works essentially the same as a small dialog picture
+(above). A large dialog picture is a 72x72 graphic, meaning it can be split into four 36x36
+parts. Thus, put each part in the sheet as if it were a small dialog graphic, in the order
+top left, top right, bottom left, bottom right. This takes a total of 8 slots.</p>
+<p>To use this graphic as the dialog picture in a dialog (created by a special node),
+put 13 in the Picture Type field. Add 1000 to the slot the left
+half is in, and put that number in the Pict spot.</p>
+
 <p><b>Item - </b>An item graphic takes up one 28 x 36 slot. Add 1000 to the slot the
 graphic is in, and put that number in the Item Picture field on the editing items window.</p>
 <p>To use this graphic as the dialog picture in a dialog (created by a special node),