Uploading pic

Hi, I have this problem with uploading images: I try several PHP classes for upload of files on server ( pictures in my case ), but if some user only renames some file as ".jpg" the script uploads the file even the file format is not .jpg - i.e. - file "test.mpg" - the user renames it to "test.jpg" - and the script upload it on server. Of course after this the website don't show any pi - it shows like pic does not exists. I need also to control the dimension of pictures ( in pixels), size in kb and type of files ( .gif, .jpg ). Is there a way to do this with some script ? Thanks, Rosen