im sure youll find stuff like this in the custom mods forum but the basic rule is something like this.
Code:
{if $cat eq "1" }
[img]{$ImagesDir}/yourpic_1.jpg[/img]
{elsif $cat eq "2"}
[img]{$ImagesDir}/yourpic_2.jpg[/img]
{elsif $cat eq "3"}
[img]{$ImagesDir}/yourpic_3.jpg[/img]
{else}
[img]{$ImagesDir}/yourpic_normal.jpg[/img]
{/if}