Borrowing the image without permission is something a lot of us do everyday for personal use. While this is technically not legal, its pretty much accepted.
Borrowing and using the image on the front page of a well established commercial entity - for the explicit purpose of promoting and selling - is not considered acceptable. This is a risk that I'm sure the Brooks folks evaluated before they chose to use it. They (most likely) decided that the time and trouble it would take for you to fight this wouldn't be worth it to you....that is, if you ever even became aware of it.
If you fight it too hard, they'll likely just cease and desist and thus take it down and replace it. Which leaves you with nothing. No great photo on the site and the good feeling that comes with it, nor anything else. Plus, the fact that they are a European company certainly clouds things.
I would write them a friendly letter identifying yourself as the owner of the bike and the image. Tell them you are really happy and proud that they chose to use it. But be clear that because they are a commercial entity and used it without gaining permission, you would like some recognition and possibly some compensation. Imply that you will accept some product in exchange to make it easy for them. Anything more than this and they'll just take it down and it will end there...unless you want to spend a ton in legal fees.
Good luck.
55/Rad
Former Copyright Infringer