Has anyone figured a way out to get FM to detect when a user drags and drops a new image into a supercontainer file? I need present the user a pop-up after the event, but only way I have been able to figure it out so far is to just disable the drag-n-drop feature. But the users really like that feature..

Long answer: There is a very, ver complicated way, which has to do with using noapplet in your webviewer, scriptmaster's trigger script module, and using a calculation to detect when the web viewer's url changes to a local filesystem url and then triggering an upload using the SC Companion plugin, but it is very unreliable, only works with certain filetypes, and may launch applications on your user's machine, give error messages, or cause slowdowns.

Short answer: No

What is it that you need the popup for?

Need to detect when an images is added, and if the user is done adding images to launch an email with the images as attachments.

I would recommend adding a button for your users to click, labeled something like "I'm finished uploading" which will trigger your script. It should be clear to users, and you won't have to worry about it accidentally triggering if they have more files to upload.