Flickr has changed their format, so there's two possible ways to do this.
On the old school version, somewhere at the top of your photo (not in your photostream) it will say "All Sizes." Click on that and find the size you want to post. There will be choices like Square, Small, Medium, Large and Original. Once you find the size you want to post, go down to the bottom of that page and find the address that ends in 'jpg.' Copy that, paste it into your message and surround it with img (
) tags.
On the newer version (which I'm not sure I like) you have to click on the photo, then go to 'Actions' in the upper left corner of the photo. Click on it to pull down a menu. Choose "View All Sizes" then determine which size you want to post. Right click on that image and choose "Copy Image Location" then paste into your message using the img tags.
Hope this helps.