If it's on a website anywhere just click on the little landscape square button at the top of the reply message box and insert the URL where shown. If it's only on your computer then scroll down below the message box to the 'Manage Attachments' box, click on that then upload pics (max of 3 at a time) as instructed.
Easiest way is to upload them to a host site like www.photobucket.com (Free to set up and use) then when you hover your cursor over the uploaded pics it will automatically give you the option of 4 codes below the picture, just left click on the bottom one (IMG code) and paste it in your thread here. No limit to the number of pictures you can paste at a time.
Or..... click on 'Go Advanced', then click on the paperclip icon at the top, this will open a small window, click on Browse, this opens the files on your PC, find and select the picture you wish to upload and click Open, this will enter the code to it in the little box, click Upload, close the small window, then click on the paperclip again and finally click on the dropdown code displayed. (Phew....!)
If it fails to upload it's because the picture size is too big. Hence the first option with photobucket.