If you wish to post a pic that is in your personal photo library on your computer,
at the bottom of the page click "post reply" you will then see at the bottom of that box "Go Advanced". There is a row of icons across the top. Click on the paper clip. (scroll across and you will see "attachments"). This will bring up another window with 5 boxes and the word "browse" beside each. Click browse which will let you go to your personal library and just double click on the photo that you want to use. You can download up to five pictures. Once you have the pictures you want listed in the boxes, click "upload now" give it a minute until it shows them uploaded. close that window and you will be back at the Post Reply box. Click attachments (paperclip) again and then select the pictures that you want to include. It sounds harder than it is.
For posting from a photo sharing site, read the how to post a pic sticky on the Forum Site Help and Support menu.