Here is what I do to post an image from flickr:
1) Go to flickr and select the photo
2) On the photo page go to the bottom right of the photo and click on the download photo button
3) From the little menu that appears select "all sizes" This opens a new window where you can pick the size you want from the selection on offer and the page will display it once clicked. Click on the size you want to display.
4) Right click on the photo and get its address. The wording for this will vary browser to browser - eg on opera it says "copy image address"
5) With the image address in your copy come to the forum
a) Either when making a new thread or click on "post reply" then use the IMAGE button in the tab below the title and above the text window. This will insert an image tag into the post like this
[ img ]
without the spaces. You then paste the url you've copied after it and then click on the IMAGE button once more
[ img ]
https://live.staticflickr.com/65535/49553335946_5f82155905_b.jpg [ /img ]
b) Manually type in the image tags (as above without the spaces). The Image button is purely a handy function to speed things along or if you forget the code.
Result