I think this is as expected. Copying and pasting images in Decker (including pasting as a canvas) seems to only work with images in Decker's internal image format (you'll note if you try to copy an image out of Decker into something else it comes out as a bunch of text).
I think dragging an image into Decker and then copying and pasting it from there into a canvas is certainly not an unintended way of using Decker, like there is nothing wrong with doing it that way. But another method is using the read[] function. With a canvas named canvas1, try this in the Listener
canvas1.paste[read["image"]]
(you'll need to press shift+enter after typing it to make it run)
It works a bit differently, since it doesn't resize the image to fit, and it doesn't apply dithering. By default it converts to Decker's 16-color palette, but there's some options listed in the manual for having it work differently. If you want to bring in images using Decker's characteristic dithered 1-bit style, then the way you're doing it is probably the easiest.