Sure, you rightclick on the image that you want to have copied, and you select the "copy this link" option, then you paste it in the image box in the reply when you click on image button...
DGN is very very picky about the type of pictures it allows, so not addresses ending in HTML or pictures of links that actually open up other pages. (I usually have to try a bunch before I get one to work) images that are jpg.s seem to work pretty well. But no hyperlinks or images then when clicked on will open whole other sites...
(I am still figuring out some of this myself)