Just treat your arrays as queues.

$img = shift @imgs;
$link = shift @urls;

# print statements here

push @imgs, $img;
push @urls, $link;
[download]

While I'm at it, I'd get rid of the temporary array, too:

while ($c = <DAT>) {
    ($id,$image,$url)=split(/\|/,$c);

    if ($type=~/^internal|partner$/) {
        push (@imgs,$image);
        push (@urls,$url);
        push (@des,$desc);
    }
}
close DAT;
[download]

I'd also point you to die and $! and maybe even unless in your open statement. But enough for now. :)

Update: Unless this program is run persistently, it won't work. Hmm -- if this program starts up again for each image, this method won't work, and you'll have to keep track of which image to serve in a file somehow.

In that case, I'd drop the randomizer and keep a counter file around. merlyn has some examples in his Web Techniques columns. Open the temp file, get the number, check to see if it's a valid index into the array. If not, reset it to zero. Serve the appropriate file, flock it, and write the incremented number to it.