Better off just doing a preload of all of them (pretty small anyway), you can't do anything from the server side, short of adding an Apache module or something to check that the client had requested and been sent at least half of the images. Or you might be able to do it with Perl and server logs, provided you have permission to change the format of your server logs and that Perl has permission to read them... but this is very silly. I think you're better off just preloading all and not worrying about it. And now for something completely different.

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10 poke53280,a:poke53281,a
20 ?"c64 rules ";
30 a=a+1:ifa=16thena=0:goto 10