You're using the right thumbcode alright, but it's just came out as text I think you might need to be a premium member to allow images in comments. If I get a chance, I'll make a Plz account out of it so you can use it in comments.
Yeah I didn't make a plz account for the banana one so I used the thumnail code : thumb358392588 : without the spaces. I'm not sure if this works if you're not a premium member or not though. Give it a go. You can check it it works by using the Preview button on the comment.
I think you may have to be a premium member to be allowed to used images in comments though but I'm not 100% sure.