OK... Try this...

When you are in your flickr album and the photo is displayed as the size you want, right click on it, and select "Properties". Your other menu items may be different than in the pic, but Properties will always be there somewhere.



Then, in the box that results, you will have the "real" address of the image. Copy it, then paste it in between the tags generated by pushing the "Img" button above the text box when you're creating a message...