I see this asked quite frequently so I'll post what I think is the easiest method. There is no way to share a picture directly from your computer (unless you're running a server in which you wouldn't be needing to read this anyway) so you have to upload your picture somewhere that IS a server.
Many people recommend photobucket which does work but requires you to sign up for a free account. Instead I recommend using tinypic.com. No sign up is required and it gives you a link instantly.
Here are some easy to read instructions with images:
1> Use your browser of choice to visit
http://www.tinypic.com. The screen will look like this:
2> Click on BROWSE and navigate to where the picture you wish to share is stored on your computer:
3> Click on UPLOAD NOW:
4> It will then give you some words to type:
5> Type the words in the box:
6> Click on UPLOAD NOW:
7> It will then give you a few links that you can copy/paste (or write down and type out):
8> You'll most likely want to use the one that has [IMG] and [/IMG] on each side of the link. This is how message boards like this know that you are posting a picture. Just copy/paste the entire thing (including the [IMG] and [/IMG] tags) in your post and it will show the picture there:
If the picture you want to share is already on the internet then you don't have to upload it anywhere.. instead just type the following:
[IMG]http://thewebsitenamewherethepictureis/picturename.jpg[/IMG]
If you don't know the address you can usually right click on the image and select "copy image location" and then paste that in your post. then add [IMG] just before the web address and [/IMG] just after it.
For example, typing this: [IMG]http://www.framingorecords.com/skuzzy/skuzzy.jpg[/IMG] produces this:
Don't ask how I was able to insert the [IMG] and the [/IMG] tags in this post and have them actually appear instead of just putting up the picture.. It's a secret!