Currently, there is no way to delete your account, but there will be in about two weeks. In the meantime, you can make it appear as if you are not a member. To do it, go to your Edit your Profile and just delete everything associated with it, like the picture and the text. Then go into the Account tab and remove the check from the box for PrettyThin. This will remove you as a member. You can then come back at any time, put that check back, fill in information, and you are a member again.
This will all change once you can actually delete your account.
You are 100% responsible for anything you post on PrettyThin. At the time of writing this, there are over 33,000 members of this site. PrettyThin does not have the resources to delete something you have posted and no longer want up. To delete it, log on as a member and find the topic of image and you will find a "delete" button next to, under, or above it, depending on what it is.
I'm working on it, but a lot of the problem has to do with the platform I'm using. If you want, just send me the image and I will upload it for you. It won't be associated with your profile if we do it this way. I tried in the past, but it's just too hard. Just wait a week and try again. It should be fixed by then.