Put your photo onto flickr (or photobucket or whatever photo site you use) then copy it's URL.
Just above the text in your reply panel you should see (next to the smiley) a button called "insert/edit image".
Click on that.
Paste the URL you copied into "image URL" box and click "insert"
Before you post, click on the image and 'drag' the corners until it is 400 or less pixels in width.