Perl is your friend. You can write a Perl script that returns image data instead of text data.

Code:
#!/usr/bin/perl -T

use strict;
use warnings;

use CGI;

my $filename = int(rand(2));

($filename == 1) ? ($filename = 'tux2.png') : ($filename = 'tux.png');

if(open FILE, "<", $filename) {
	print CGI::header('image/png');
	binmode FILE;
	print while(<FILE>);
	close FILE;
} else {
	print CGI::header();
	print "Error: $!";
}
Now if you use an IMG tag and put the .cgi script as the URL, if you refresh this page and it should show a random picture.

<img src="http://chwombat.net/cgi-bin/picture.cgi">

gives

<img src="http://chwombat.net/cgi-bin/picture.cgi">