Why ResizeImageOnUpload Stops Working Intermittently

The donor report described a frustrating but very common pattern: images still uploaded, but the resize plugin stopped processing them reliably. Some files worked, others failed, and switching PHP versions changed the behavior only temporarily.

What Makes This Problem Tricky

• The original file still appears on the server, which makes the upload look successful.
• The resize step fails silently or only for some files.
• The issue may depend on server libraries, image dimensions, memory, or hosting restrictions rather than only on file size.

Core Plugin Pattern

if ($modx->event->name != 'OnFileManagerUpload') { return; }
require_once MODX_CORE_PATH.'model/phpthumb/phpthumb.class.php';
$phpThumb = new phpThumb();
$phpThumb->setSourceFilename($filename);

What to Verify

• Whether the target folder path from the media source resolves exactly as expected.
• Whether phpThumb can allocate enough memory for the source file dimensions.
• Whether the image extension and MIME type match the allowed upload image list.
• Whether the host uses LiteSpeed or another stack with stricter execution or filesystem behavior.

A useful clue in the donor case was that the original file still appeared on disk. That usually means the upload event fired, but the thumbnail generation or file write step failed later in the chain.