You might want to take a look at strtok (http://www.cplusplus.com/reference/clibrary/cstring/strtok.html).

Not quite. You just store an id indicating which stylesheet to load. When loading the page you would then check for the id in the cookie, and if the cookie does not exits use the default style. You...

Yes, but AFAIK it would require a little scripting. You could save the style in a cookie, then select a stylesheet based on the value when the page loads.

It seems that you are not increment the id maybe add an increment to your loop? IE:
$id=1;
$count = mysql_num_rows( mysql_query("SELECT image_id FROM imagetable") )
$data = mysql_query("SELECT *...