The image I have just posted under Shadows and Reflections is 800 px wide for your info.
To post as an attachment, choose and upload your file in the 'upload attachment' section which is situated under the preview/submit/..... buttons at the bottom of the message box.
You have to write something in the message box too as it won't accept an attachment without an accompanying post. Just a word will do.
You can still do it the 'old' way and link to a bigger image using the Img button at the top of the message box but I believe the forum will downsize it to about 1000 px anyway.
Get in touch if you have any difficulties and happy posting
