Under the text box that you use to make a post is a section titled "Additional Options".
You press the button labeled "Manage Attachments".
Once that screen opens, you click "Browse" and select the file you want to upload.
Click "upload" and wait for the desired file to complete that process.
Once you've finished attaching the files, "Submit Reply" as normal.
FWIW, there is a size limit for pictures.