if you want your pic to be your avatar, you could go to "My Account" then click "Edit" tab and look for the "Upload Picture" portion, click browse then select desired pic for avatar use...

if you want your pic along with your reply/post then you could follow these example;

1. The only way to make your pic available whenever you made a post/reply on forums is to upload it on a file hosting sites such as www.photobucket.com (this is only an example coz there are a lot out there offering such services). As per my example, go to the site, if you don't have account, make one, then find the portion for "Uploading" images/pic. Once uploaded, your pic/image are now readily available online.
Remember your photo/pic/image Link, it's normally your assigned URL plus the image filename when you uploaded it.

2. then use HTML tag (i.e. img) like the example below;
< + IMG SRC="(your picture/image link here on photobucket)"
ALT="(your desired alternative text, incase photo doesn't appear)"
TITLE="" + > (removing the "+" sign)

sample:
http://i123.photobucket.com/albums/mm8/(your username here)/ + (photo filename here).gif or jpg,

also, say your photo/s filenames are;
pic1.gif, pic2.gif and pic3.gif

your username at photobucket is;
username

then your url at photobucket is;
http://i123.photobucket.com/albums/mm8/username/

if you would like to make pic1 viewed on your post use these html tag;

< + IMG SRC="http://i123.photobucket.com/albums/mm8/username/pic1.gif"
ALT="My Picture Online"
TITLE="" + > (removing the "+" sign)

then if you want pic2.gif, just change pic1.gif to pic2.gif, like this;

< + IMG SRC="http://i123.photobucket.com/albums/mm8/username/pic1.gif"
ALT="My Picture Online"
TITLE="" + > (removing the "+" sign)

and so on. now, you could easily maneuver on changing/replacing your pictures/photos on your post. just remember, if you desire to make diff. kinds of photos on diff. post, try uploading all those photos, then just change the name in your html tag.

also, you could put the tag on the signature portion on the "Edit" tab on your "My Account" to make it as your signature photo.

hope it helps...