Hi abusalah,
I'm sorry for any confusion, but I'm not clear on what you want to do.
If you want to have random custom images, you can do that by uploading your images in the Appearance > Header area and choosing Random for your images.
If you want to have a specific custom image for each Page or Post, set a featured image for each Page or Post.
If instead, what you want is a mix of the default images and your custom images, try downloading the images you want from the default images (wp-content/themes/twentyeleven/images). You can use either an FTP program or your web hosts File Manager to download the files to your computer. From that point, you can either upload them along with your other custom images, or set them as default images on specific Pages or Posts.
Hope this helps!