The GD2 string UBBThreads is producing certainly works but not the imageMagick string.
You're welcome to open the php script yourself and skim through it. The scrip is simple, quite small, and easy to quickly get through. Its located at:
/libs/image.inc.php
In that script, the same basic string format is used for each:
$exec = "\"{$config['CONVERT_PATH']}\" {$config['FULL_PATH']}/tmp/$sourcefile $thumb {$width}x{$height} $quality {$output}:\"{$config['FULL_PATH']}/tmp/$sourcefile.$type\" 2>&1";
Remember though, the setting which UBB.threads use are the most basic, with the pure intent of covering the broadest type of images sizes and their content. A photo of the grand canyon vs a photo of a starry night vs an animated gif vs a family gathering vs a screenshot or a transparent company logo... of which may all produce different results.
Regarding UBB.threads, if your intent of using the image attachments feature is to share visual images with others on the internet, it just works as intended. But if you need the attachments to be guaranteed untouched, or you have a batch/package of many images, your best bet is to put them all in to an archive (such as zip or rar) and attach that file.
Further reading:
https://www.sitepoint.com/imagick-vs-gd/(nothing specific other than it being the very first link in google when searching for im vs gd)
And of course:
PHP & GD @
https://secure.php.net/manual/en/book.image.phpPHP & ImageMagick @
https://secure.php.net/manual/en/book.imagick.php
