Best way to do it without using a outside source like phot bucket is to click on the photo album icon in the orange tool bar at the top of the page. From there right below your sign on name is upload photos, click on that.
It will bring up the browse feature which allows you to look through photos on your computer. Click browse and look for the photo that you wish to upload, then click upload.
After the picture is done uploading to your photo album you can view the picture, click on it thus making it larger as opposed to thumbnail.
Right click on the picture and hit properties.
You will see a address that begins http. Highlight that entire address. right click again on the highlighted area and select copy.
Then go to your thread that you wish to put the picture into. type the following
after the [img] right click again and paste the address.
after the address is inserted type and hit submit.
Your picture should then be posted in the thread. Kinda like this
I left out the / in the second brackets so you could see what it looks like.