You can do it two ways.
One you can go to photobucket as described by LLL. Many here do that.
I do it this way, I download the picture from the internet onto my computer. When I want to show the picture
1. Go Advanced (button option to the right of submit reply at bottom of where you post)
2. click on paperclip icon at top of advanced posting.
3. A small window will open (called manage attachments) and I hit browse (browses my computer) I select the picture I want to post.
4. Click upload.
5. The window will let you know when your upload is complete and I click close the manage attachments window.
Finish the post and click submit. Your picture will show on your posting.
I do it this way because I post a lot of pictures for missing people. So, two ways you select which is best for you