Click on the "Attachments and other options" text that appears below the message box when typing a new post/reply.
Go to Attach and click Browse which should open up the pics in your computer files.
Select the one you want and click on Open
The pic will then appear below your message when you click Post, or you can Preview if you wish
Hope that helps
PS You cannot add more than two or three large pic files at a time, but there is no need to resize as we used to have to