If the image is uploaded to flickr here is another way with a few optional steps removed:
Either find the image in your photostream or the set and left click with the mouse to display the image you wish. Then with the mouse over the image, you can right click once to see all the size options, such as medium, large original. Move the mouse to the size you want, then right click once and a small window opens. At the bottom, "Copy Image Location" appears. Right click once on that and you have the image.
Then what I do is return to the thread window I am making my post in and I manually type [IMG] and I drop my mouse right after, right click and then paste the image. To complete the process, I manually type [/IMG] and then you can see your efforts by previewing the post.