Use a switch statement, I forget the exact syntax in Torque, but something like this should work:
%healthBitmap = "";
switch(mFloor(%health / 20))
{
case 5: %healthBitmap = "<bitmap:Add-Ons/server_totalrpg/100%health>"; break;
case 4: %healthBitmap = "<bitmap:Add-Ons/server_totalrpg/80%health>"; break;
case 3: %healthBitmap = "<bitmap:Add-Ons/server_totalrpg/60%health>"; break;
case 2: %healthBitmap = "<bitmap:Add-Ons/server_totalrpg/40%health>"; break;
case 1: %healthBitmap = "<bitmap:Add-Ons/server_totalrpg/20%health>"; break;
default: %healthBitmap = "Dead";
}
commandToClient(%client, 'bottomPrint', ... %healthBitmap ... );