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.

Millie is correct; Decker doesn't attempt to manipulate native image data in the system clipboard; it's very difficult to do that sort of thing in a cross-platform fashion, especially once web browsers are added to the mix.

Decker uses a custom text encoding for information stored in the clipboard. If you're interested, it's described in The Decker Document Format. This is also the same format for images you can obtain with the ".encoded" attribute of image interfaces.

Keep in mind that anything you find yourself doing repeatedly in the Listener can be moved to a button script, and parameterized with other widgets. Use Decker to build tools to remove tedium from working on your projects!