As a registered user, you can post pictures using a picture hosting site. There are several free ones.
If you use imgur,
create a free account
Click <NEW POST>
Follow the directions to upload.
Once a picture has been uploated, go to the top right and mouse over the bubble with the first letter of your user name. That gives you a drop down.
Select "Posts"
That will show all your images. Click the image you want to post, then to the right at the top, copy "GRAB A LINK" address.
Then go to your post and click the "insert Image" tool on the tool bar. (It is located between the LINK tool and the SMILEY tool.)
Paste the picture address into the resulting popup, then click "Insert".
If you use another hosting site:
Once you have uploaded a picture to the hosting site, copy the web address of the picture. Note: picture web addresses must end in .jpg or .gif
Then go to your post and click the image tool on the toolbar next to the smiley tool. Click the link icon in the popup. (Looks like a chain.) Paste the picture address into the window. Then click the INSERT button.