I've just renewed my passport online and wanted to pass on how simple this procedure is!
If you're not confident that you have a suitable photo to download, do as I did and go along to your local Snappy Snaps or similar. They give you the four printed photos as usual; an email with a JPG file attachment of your photo; and via their Home Office-approved technology a code which you quote on your on-line form.
When filling in the short form you just enter your code and voila your passport pic pops up!
The photo of me is still grim though - par for the course! 
He never gives up being stupid.

