Text editors

For open full-size in new window -

Insert the image, and drag it by corner to proportionally resize it small enough to suffice.
Find the image location -- Right-click on the resized image and select "copy image location".
With the image still highlighted, click the editor Link icon, and paste the copied image location into the address field.  Select open in new window (I find the same glitch you do with this - & often have to go into raw html view to confirm that the target tag is inserted properly).
Since it is not necessarilly obvious, I add the text "- click on image to open full-sized in new window -"  beneath the image.

Users of course always have the option of doing essentially the same thing without this fuss, by right-clicking the image, and selecting "view image" - which will open the original file of the resized image.  Depending on their browser settings, this may open in the same window, or in a new window or tab.