I can get a pretty consistent (bad) behavior.
If I drag a image from a webpage to evernote. I can view the image in the evernote, but I cannot resize it.
However, if I drag the same image to my desktop which save the image as an jpeg file, then I drag the jpeg file to evernote (web/mac/windows). I can resize the image.
I think if you just drag the image from website, you are not save the image to evernote, you just save a link(I guess) to the note. So you only view but not edit(i.e. resize)