[650]

You can click 'go advanced' below the box you type in, then click 'manage attachments' then click 'choose file'. Then select the picture, then click 'upload'

Although every time I do this, regardless of file format or file size, I get told it's an invalid image. This site is archaic for things like this.

So your alternative is to sign up to Flikr and upload, then click the share button and copy the 'BBCode' and paste it in a response here.