Can the user import files from his computer, or just from the library where he has a selection of images created by you?
In case of the former, you will be needing the FileReference class and this will be a theoretical approach, because I'm not familiar with it. :)
Basically, what FileReference does is allow the user to browse his local drive and upload a file to the server. This class is present in both AS2 and AS3, and there should be plenty of information in the Flash Help files.
Once the file is uploaded to the server (FileReference.onComplete), initiate the MovieClipLoader class and load the image into an empty movieclip (the filename you can get from your FileReference class).
Then add the text onto the picture and create a printJob (PrintJob class and PrintJob.addPage in the Help files)