Dear Paint.Net masterminds, I've been a user for many years, enjoying the app, but this has always driven me nuts.
I've dragged thousands of images onto the Paint.net window and NEVER, ever, did I want to add them as a layer. Ever. If I do, I'd go to the Layers menu or something similar. I never add layers. I only work with simple images. But Paint.Net hijacks this trivial drag-and-drop operation which is used in virtually every app out there, and interrupts it with its prompt.
You have to please the majority of users, not the select few who actually drag-and-drop an image to add it as a layer. I bet it's a small percentage of your users.
Here are possible solutions:
a. Add a checkbox "Don't ask me again" and add a tip "To add as a layer, use Layers menu". Or,
b. Simply assume users wish to open the picture they drag and drop onto the window?
c. Check if I'm holding down a special key such as <Shift> or <Alt> while dragging and dropping a file, and act differently - that could be the "open file" action.
After all, I'd think that I can drop an image onto the Layers toolbar/panel or use another trivial way of adding it, without breaking the most obvious way to actually open an image.
Thank you for all your hard work, and cheers!