About half the time I use the application is for creating images, and half the time to simply create a before-and-after/side-by-side style images for work.
I will typically screen shot the before image, create a new image in Paint.NET which cleverly knows the image size in the New dialog, but when I take a second screenshot what I want to do is paste and be prompted to expand the canvas to include both images.
What I envisage is basically choosing the current Expand Canvas to fit, and then by default it will show the following mock-up with the image in the center, and if the center option is selected then it just does the automatic expand to fit functionality that it currently does, and pastes your copied image right over the middle (again, existing behavior). BUT if you choose to anchor the existing image to the left/right/top etc the expansion would fit the images side by side:
i.e. original image was 445 x 133, and the new one is 255 x 100. Using the (proposed) anchor-when-paste functionality, and selecting to anchor the image to the left, the canvas will expand the width to 700 (445 original + 255 pasted) and set height to 133 (no change, because the pasted image is shorter than the original).
Thanks for your consideration!