when you create new user page, there may be one Image control.. in that set imageurl as ur default image, such as default.jpg..
now when user want to choose any other image , he wil select any image by using image upload button..
but at the time of creating account if user dont want to upload image then this default.jpg will be selected as his/her display pictr.
so in that case when u fire insert query , there may be a field like "imageurl" in that field if user selected any image then insert that uploaded image, If not then insert default.jpg as user's imageurl field of table..
hope you get this !