It's because of a mismatch between Flickr's forum sharing format and the site's 'smart' software. You are using 'BBCode' format which is aimed at rather simple forums that are not able to parse links. If you use the link from the 'Share' option and just copy this into your post without any additional tags, it will turn up as a single image:
I used this option:
The long story:
The 'BBCode' format includes several links. One to show the image between IMG tags and two others to enable someone to go to the same image on Flickr, between URL tags. On a less smart forum board these will be processed as the HTML code that they are and will show up as an image you can click on and a link underneath it. The VBulletin software that PPRuNe uses sees at least two of these as links to Flickr and automatically 'adjusts' these as shown in your post above.
I have removed all the square brackets in the code snippet above and have added some hyphens to prevent these being processes as links. The three links in the code are in bold font.