It probably depends on the nature and tone of the complaint you've received, and on where you've taken the picture from. You've got three options: respond to the complainant, pass it on to Geograph, or ignore it. In any case, remember that there's nothing wrong with taking pictures from places with public access as long as it doesn't infringe someone's privacy in a very specific manner.
If the complaint is a reasonable request, you may wish to discuss the matter with the complainant directly - they may well be happy with your picture and text once they get to know you a little and realise you're a friendly if slightly excentric square bagger rather than a horse-rustling paedophile terrorist.
If the complaint is irate or makes unreasonable claims for privacy, you may prefer not to let the complainant have your email address. If you don't respond, they can always complain to Geograph. Alternatively, you can forward the complaint to [email protected] , who will deal with it on your behalf. It helps if you present your side of the story and say whether you've taken the picture from a public space, a Public Right of Way or from private property without statutory access. Please make sure that the email address of the complainant shows in the body of the forwarded message.
On the other hand, if you do want to deal with an irate complaint by replying directly, it is usually a good idea to wait a couple of days to diffuse the heat of the moment.
If the complaint is about the text rather than the image, it is often possible to find a wording which will pacify the complainant without distorting the meaning of your text.
Once you have pressed the "I agree" button your picture is in the system and subject to the Creative Commons Licence.
If the picture has not yet been moderated, you can requested for the image to be rejected, and you will be asked to give a reason. To "self moderate" in this way you click on the "Change Image Details" link under the picture: you should see an icon for "reject".
Once the picture has been moderated and accepted you will probably need to give a very good reason for the image to be removed from display. Images may be removed from display if they were taken illegally, or, for example if a landowner objects to a picture taken from a place without public access. This process of removing from display is sometimes referred to as "vaulting" as a copy of the picture remains in the "vault": a file storage area.
Sometimes pictures get relocated if they have been uploaded to the wrong grid square by mistake. This is often the case with images from the very early days of Geograph, when there was no or only scanned out-of-copyright mapping available.
Very occasionally, a (first) Geograph gets reclassified as a supplemental image* if there had been a clear error in the original classification.
Also rarely, images are withdrawn or vaulted for a long period because they weren't taken from a place with public access and the land owner has complained.
In all these cases, the next uploaded geograph (if any) will take the point, and if that was yours, then you'll find a miraculous extra point on your profile.