Changing image on header and sidebar

Link might help..

#header {height:250px;background:url(‘http://aircraftprofiles.dk/wp-content/uploads/header.jpg’) no-repeat;}

#footer {clear:both;width:100%; text-align: center; height:250px;background:url(‘http://aircraftprofiles.dk/wp-content/uploads/footer.jpg’) no-repeat;}
These images are in uploads folder.. I think sandbox theme provide option for custom header and footer imaages..

As suggested, take a look in your themes options, you may be able to alter them there.

Otherwise just upload and overwrite new versions of the image you want to replace (links to some of the images given above).