You don't have to use photobucket you can use the site if your not uploading huge pictures,
First you,

Push post new reply,
Then you

Click 'choose file'
You locate the file on your computer and select it
Then your picture will show by the red arrow

You click 'add attachment'
It will look like this

You then write your post (where the blue arrow is)

Once you reach the point you want to put the photo in you click the 'insert into post'
And it will look like this (red arrow)

Once you've completed your post click the 'post reply'