If the image has not yet been moderated go to the 'Change image details' page and click the [request rejection of this image] button, explaining it is the wrong image.
On a moderated image, open the 'Change image details' form and, in the box near the bottom of the page labelled [Please describe what's wrong or briefly why you have made the changes above...] ask for the photo to be rejected and give your reason. Don't forget to click the box marked 'Bring this issue to the attention of a moderator' before clicking [Submit changes].