The Problem here lies within the Plugin, which utilizes the deprecated Function image_resize. I’m not 100% sure that this is the only Problem, but try rewriting the Plugin using the new [WP_Image_Editor][1] Class.
The next Step to check is if the Plugin can write the files, and if the directory permissions are set correctly.
See Line 128 of your code:
$new_img_path = image_resize( $file_path, $width, $height, $crop );
This line returns a String if executed correctly, or an WP_Error Object on Failure. I’m pretty sure this is where your Plugin goes wrong.
If you do not want to rewrite it (which I highly recommend), try writing a small Plugin to find out if your image_size works as expected, or if you have to change your Server’s settings.
For the Image functions to work, you have to have your Uploads Directory writeable for your Plugin Files.