Quoted from the website:
Maybe you know this problem. You visit your favourite forum and want to show a picture in a post. Flickr generates HTML code and this code includes the picture and a link to your flickr website. This is exactly what you need, but most of the message boards don't allow HTML tags like <a> and <img>. You have to use the Bulletin Board Code (BBCode) and must rewrite the flickr HTML code.
Here is my solution. Just copy the flickr HTML code into the text field and click the Convert 2 BBCode button. The HTML code will be converted into BBCode and you now can use it in your post. This works only with the flickr HTML code!

Link: http://www.derwunsch.de/f2b/index.php