I found the issue and created a quick patch that you can update the file on your own server with right now. Though, It is best to just wait until the next version of UBB.threads is out, since it will have a more detailed code block inserted. This code is only needed if you are using ImageMagick. GD2 does not require this attribute at all.
I've tested this with and without GD2. GD2 presents no visual errors, and still processes the images as intended -- Though, with the attribute added, GD2 may create an warning entry in the error log such as, "unrecognized command "-resize" but image was processed anyways." But I havent checked this. And thats only an assumption.
in forums/libs/image.inc.php
EDIT 2018-02-21:
IF YOU PREVIOUSLY MADE THE FOLLOWING CHANGE TO YOUR IMAGE.INC.PHP FILE, REVERT THE CHANGE OR GRAB A FRESH COPY OF THE FILE FROM THE ORIGINAL ZIP ARCHIVE. THEN SKIP THIS STRIKED-OUT SECTION, AND PROCEED TO THE "MAKE THIS CHANGE" SECTIONFIND (towards the end)
$exec = "\"{$config['CONVERT_PATH']}\" {$config['FULL_PATH']}/tmp/$sourcefile $thumb {$width}x{$height} $quality {$output}:\"{$config['FULL_PATH']}/tmp/$sourcefile.$type\" 2>&1";
REPLACE WITH
$exec = "\"{$config['CONVERT_PATH']}\" {$config['FULL_PATH']}/tmp/$sourcefile $thumb -resize {$width}x{$height} $quality {$output}:\"{$config['FULL_PATH']}/tmp/$sourcefile.$type\" 2>&1";
MAKE THIS CHANGE:FIND (towards the end)
if ($type == "thumb" || $type == "medium") {
$thumb = " -thumbnail ";
} else {
$thumb = "";
}
REPLACE WITH
if ($type == "thumb" || $type == "medium") {
$thumb = " -thumbnail ";
} else {
$thumb = " -resize ";
}
The ImageMagick resizing issue seems to have been missing from the code long before I came on board with UBBCentral. So I guess its now finally squashed. Digitalgas, thanks for posting about it, to finally give it some needed attention!