You say that your photos are already in flickr? If so, you should be good to go.
Once you are logged in to flickr and you see the pic you want to post, look for the button that says "Share", above your pic.
Click "Share" and a menu will drop down with some options. There are different ways to proceed now, but I reckon the following method is the easiest:
Choose "Grab the HTML/BBCode", and another drop down menu appears.
A couple more options to go:
Picture size - Medium 800 is pretty safe. Your picture will also function as a link to flickr, so viewers can have a better look at it, if it's much bigger than that.
Lastly, a little bullet point that chooses between HTML and BBCode. This changes the gobbledegook instructions for advrider to display the pic. You don't need to worry about it, but I would choose BBCode.
Now, what you need is this code, displayed in the box. Click on it and it will be highlighted. Copy (the "⌘" key on your mac + the "C" key), and now Paste into your advrider post (the "⌘" key + the "V" key).
The code will appear as text in your message, but click "Preview Post" and you should see your pic. If you're happy, you can now post.