Consider working with SPL's fileinfo and iterator classes (i.e. SplFileInfo, DirectoryIterator and FilterIterator, CallbackFilterIterator, etc) to build something a little more resilient.

Example:

$directoryIterator = new DirectoryIterator($directoryPath);
$imageFilter = function (SplFileInfo $splFileInfo) {
    return preg_match(
        '/^screenshot[0-9]{1,3}\.png$/',
        $splFileInfo->getFilename()
    ) ? true : false;
}
$filteredImageIterator = new CallbackFilterIterator(
     $directoryIterator,
     $imageFilter
);

foreach($filteredImageIterator as $image) { /* your code */ }

Consider working with SPL's fileinfo and iterator classes (i.e. SplFileInfo, DirectoryIterator and FilterIterator, CallbackFilterIterator, etc) to build something a little more resilient.

Example:

// your filter logic
$imageFilter = function (SplFileInfo $splFileInfo) {
    return preg_match(
        '/^screenshot[0-9]{1,3}\.png$/',
        $splFileInfo->getFilename()
    ) ? true : false;
} 

// get files in directory based on filter function
try {
    $directoryIterator = new DirectoryIterator($directoryPath);
    $filteredImageIterator = new CallbackFilterIterator(
         $directoryIterator,
         $imageFilter
    );
} catch (Exception $e) {
     // do something here, or don't wrap in try-catch at all if you want to bubble up exception.
}

foreach($filteredImageIterator as $image) { /* your code */ }

Note that you can build upon the filter shown here by adding things like verifying file is readable/writable, get file modification timestamps, or other such functionality exposed by SplFileInfo, which could include ability to easily create SplFileObject for working with file contents.