It all depends what kind of images they are. If it's copyrighted stuff you'll likely get an email from Google telling you to remove them, as would be the case if the images were of "naughty nature" (porn).
It also depends on how you place your ads. If you place them right beside images, Google may say that is drawing "unwanted attention" to them.
Will they still make a problem even if there is just a few mature pictures? lets say 4 out of 1000? and does paypal care about that too?
Not too sure what PayPal thinks of that sort of content, but yes, even if you have just a few adult pictures Google doesn't like it.
yes those 4 will make a problem it's ok to put adsense on imageboards but from my experience on pages with images people don't click much
If you are talking about image hosting site.. then do not use adsense with it as you can not control users to upload adult pics.