When you click "Copy Image Address", that's exactly what you're asking the browser to do: "copy the URL of this image to the clipboard." So when you hit paste, a URL will appear.
With some web setups, Chrome under Windows for example, in some web applications when you open the file picker, you can paste in this URL: the browser will download the image in the background, then upload it to the web application.
This won't work in Dynalist because there is no file picker.
Some web applications support "Copy Image" to paste: in the background the browser downloads the image to your temporary files, then uploads it. Facebook is a good example of this.
This won't work in Dynalist.
When you take a screenshot, the image literally is in your clipboard -- it doesn't need to be downloaded; it's there. Upon pasting the browser uploads it to Dynalist and voila, you have an pasted image.