You can't upload a picture directly to the forum. You need the picture to be uploaded to somewhere else on the internet and then provide a link to it in your posting.
As mentioned, Photobucket is a good free place to host your photo's. Upload them to an account there and then you will see 4 choices under each photo. Next to 'IMG code' you will see a box with some text beginning http etc. click on this text to highlight it and then press Ctrl & C (cmd & C if you have a Mac) to copy the text. Then come to this forum and compose your message and click Ctrl (or cmd) & V to paste the text into your message.
You should then find that your posting contains your photo.