You can only post photos that are uploaded to the internet. The reason being is that the HTML img tag only links to pictures with a URL.

Let's say I wanna post a pic with cute kittens in it. I go to Google Images and find a pic of kittens. When I right click on the picture, a menu will come up. I click on Properties. You'll see where it says Address (URL). You highlight the entire URL of the picture, right click and copy it. (Or click on "Copy Image URL" if you have that option) Come back here, go to post a reply, and click on the icon that looks like a painter's palette in front of a monitor. That will add the img tags to the reply field. You then paste the picture URL in between the tags.

If you right-click on the picture above and click Properties, you will see the address(URL) of the picture is

That would be what you would cut and paste into the img tags.

When you upload photos to photobucket or any other photo storage site, that will give them their own unique URL. Then you would post them in the same manner as any other picture on the internet.