i think the way you're doing it is messing it up.
this is what i usually do:
1) open image into a new tab and highlight the link at the top and copy it
2) go to my signature and type [IMG.](paste image link here, and remove parenthesis)[/IMG.] (delete the periods, i just put them there so it wouldn't code in the vm
3) hit save changes and it should work :')