You are posting pics into your avatar slot, rather than into a message!
You can only see one avatar picture at a time, so that's why you can't see any others you pasted in after.
You need to pick one pic for your avatar, get it in the small size, then paste that url into your avatar (which you did the right way with your first pic). At the moment that's a medium sized pic in your avatar, which stretches the space too large. If you want to keep that pic as your avatar, just go back to Flickr, get the url for the small size, and paste that into your avatar slot in your profile instead.
Or you can put your other pic back in too. You can change your avatar as often as you like of course!
To post images to a thread so we can see and enjoy them, you copy the url from a medium or small size pic, start a topic, write whatever you want, then click the IMG button (second from right on the menu just aboove the text box) into the actual text box of your message, paste the URL, and then click the IMG button again.
If you then click 'preview' (rather than 'submit') you can see if it worked. If it did, then click 'submit'. Or you can just click 'submit' to see if it is showing up.
If you click 'reply' to this message just to bring up a text box, you will be able to see the buttons I am talking about.