A lot of people use flickr here, with no trouble, so it sounds like
something you are doing is not right.
First - use the 'insert image' icon above ( not insert link)
Second - paste the address from the picture you are posting in the box
Third- click OK
The address is at the bottom of the page when you have the picture you want to show open ( after you've clicked the 'all sizes' button).
Using the address from the top of the page ( in the address bar) will not work.