when you get link like that
http://imgur.com/uaTRD0W , you should click on the image again to open it and copy link from there (or just right click and copy adress). For something to be embeddable with [IMG] tags, it should end with extension e.g. .jpg or .png, so you should be putting in
[SPOILER="112233"][IMG]http://i.imgur.com/uaTRD0W.png[/IMG][/SPOILER]
for
but regarding sigs:
1. For images in signatures you need toriprime/VIP,
2. Images in signature aren't placed with [img], but rather you upload the picture in signature page and then use [sigpic][/sigpic]