Addition to an integer field with a form I have a bundle of image nodes of the same content type that I create in my custom form. Each image node has a field that contains a unique reference number to that node. Currently it is up to the user to add +1 to the previous image node saved and type that in the form for the next reference number. How can I add +1 to the last saved field and use the result to populate the saved field displayed on the saved node? More advanced: If I use an AJAX form that allows users to add more fields to add more pictures on the image creation form, how could I save the images with consecutive IDs?
You can use the module Prepopulate ; latest release is dev but I use it on a production site and it works like a charm.

If it doesn't work tell me, there are two other solutions : send an arg in the URL to the form (and then use Panels or the php template) and JavaScript (I can give you the code if you need it).