The first thing is to have a place to host your pictures. I use
It is free to get a basic account, and quick and easy. Once done you can upload your pictures to s folder in your account.

When you mouse over the thumbnail on photobuxket you'll see some HTML text. Copy the bottom one that has [IMG] at the beginning.

Paste that into your post, and that is it.

If you have a direct link to a picture you've posted on the web, you can use the Img button to put the image HTML markers in your post, and just paste the URL between the img and /img