If you're on Linux, the file name needs to have the same case as the map name. Like MapName.bms is not the same as mapname.bms on Linux. If you make sure all the photos are lower case you can use this:
Code:
if(file_exists("./mapimages/".strtolower($Server['MissionFile']).".jpg")) {
echo "<img src=\"./mapimages/".strtolower($Server['MissionFile']).".jpg" />";
} else {
echo "<img src=\"./mapimages/nophoto.jpg\" />";
}