I don't think Iggy-sama can answer this question either? As for what is planned so far, I think Checku-sama said something on the line of "There will be a large variety of images pre-rendered you can choose from" and at the moment from my understanding there is no plan for user custom image, however that could very well be subject to change as it is further down the development stage. As for what images are used, I don't know what range is Checku-sama looking at, however I would hazard as guess that when this is further down the development stage there might be a poll to pull some suggestions of images that people would like to choose from, or it might be completely changed to a user customization just like avatar or signature, if it follows the latter route then you have nothing to worry about, if it follow the first route then maybe you can give some suggestions regarding that, if it get picked then obviously it would appear, if not then you have to pester Checku-sama :P