Picture files for menu items whose name begins with an underscore (if not preceded by a path) are seen as resource instead of file.

To recreate:

Choose a menu item in the menu designer
Click on the Options button for the item
On the option group that says File and Resource, choose "File"
Choose a file whose name begins with an underscore and remove the path in front of the name
Press OK
Click on the Options button for the item again
The option group now has "Resource" chosen

So for example, lets say I have a file called _NEW.BMP - the generated menu code is

DEFINE BAR 1 OF shortcut PROMPT "Create New Fiscal Period" ;
PICTRES _new.bmp

instead of

DEFINE BAR 1 OF shortcut PROMPT "Create New Fiscal Period" ;
PICTURE "_new.bmp"