A lot of people seem to be confused about how to post a photo to Ravelry. Now I can't tell you how to do it using Flickr because I'm not overly keen on Flickr and besides I already had a Photobucket account when I joined Ravelry.
So I'm going to give you a tutorial on how to post pictures to the forums and your projects/stash pages on Ravelry!
First (obviously) you need a Ravelry account and an account with Photobucket.
After you've uploaded your picture to Photobucket (which I'm going to assume you know how to do by following the instructions on their site) you'll want to proudly show off your latest FO or new yarn purchase to everyone else.
This next section is about uploading to your projects/stash pages.
Create the listing as normal, now click the photos tab and scroll down to the bottom. You should see a row of tabs labelled "flickr importer", "photobucket", "slurp from web" and "upload from computer". Obviously you want to click "photobucket".
Now, you need to enter in the user ID and password you use to log in to PHOTOBUCKET, not Ravelry! It might take a while to load or you might need to do this a couple of times - Ravelry is still beta, don't forget!
Once your username and password have loaded in, you should see a drop down box labelled "Album". You need to locate the album on Photobucket that you saved your picture into in the drop down box. Once you've done that a load of thumbnails should appear. You then scroll through until you find the picture. Click on it and your picture will appear above the line.
In the bottom right hand corner of the picture that appeared at the top of the page there will be a square with arrows in, if you click + hold that, you can drag the picture around in the box until you are happy with it.
Ravelry automatically saves any photo changes, so there is no "save" button to click.
Now, this next section is about how to load your picture onto a forum post.
Go to photobucket, it doesn't matter which album (I have several); under the first photo you should see the words "Link options". If you click this a box will appear, there are three tabs across the top, click the far right one labelled "Get link code" and in the selections look for "HTML for websites and blogs" and under that, tick "clickable thumbnail".
At the bottom, put a dot in "always".
Now click save and it'll take you back to your album.
Click in the box under the photo you want that has "HTML Thumb" next to it and then press ctrl+c to copy that link.
Go to Ravelry (this is where multiple tabs come in handy!), write your post and in the place you want your picture to appear, press ctrl+v to paste your link. It'll look like a load of gobbledegook until you've actually posted your comment, but once you have; a nice little thumbnail of your picture should be there!
Be warned: if you delete your photo from photobucket, or you move the photo to another album; it will no longer be visable in your projects/stash pages or on the forum you posted it in!