Thomas: here are the steps you would need to do to use the signatures as "variable sub forms":
1. Create a Global User variable - call it something like SIG, and set it to reset at the set level.
2. If the section where you have placed the USER ID (liley either Page Header or On Ship To 1 for a PO), create a User Variable and give it the same name as the GLobal User variable (I.e., SIG). This "defines" the value of the global variable, meaning it has its value continue through all sections in the set without having to be redefined. Set the value to the equal to the trimmed value of the USER ID field (i.e.. trim(USER_ID_4).
3. In the section where you want the signature to show up, add a variable sub form to the section and place it where you want the signature to show up. Base the sub form on the SIG global variable. I would suggest you set the sub form to auto fit and preserve aspect ratio, and to have it aligned left and bottom.
4. Create the sub forms by importing the signature images (Tools / Subform / Import Images. Before doing this, you should a) make sure each of the image files is roughly the same size in terms of height and width, and b) (very important) make sure that the image name is the same as the USER ID ... so for user TKIM the image file should be called TKIM.
What this gives you is a) a variable to hold the value of the USERID field that can carry through any sections on the report (easiest way to do this), b) subforms named the same as the USERID, c) the link between the variable and the signature to use on the form. This is relatively standard stuff in the 3.x version of Create!form, and makes maintenance a lot easier than using alternate sections. Of course, you will need to add the sub form each time you add a new user, but you would need to do this anyway to get the image onto the PO.
Let me know if you have any questions. If need be, I can walk you through this over a web session.