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In the file inc/functions_image.php the function generate_thumbnail() generates warning due to the following line:
list($imgwidth, $imgheight, $imgtype, $imgattr, $imgbits, $imgchan) = getimagesize($file);

the construct "list" accept only numerical array and the function getimagesize will generate something like:
Array[0] = Width
Array[1] = Height
Array[2] = Image Type Flag
Array[3] = width="xxx" height="xxx"
Array[bits] = bits
Array[channels] = channels
Array[mime] = mime-type


So $imgbits and $imgchan are empty.

a really simple solution could be

@ $file_size = getimagesize($file);

$imgwidth  = $file_size[0];
    $imgheight = $file_size[1];
    $imgtype   = $file_size[2];
    $imgattr   = $file_size[3];
    $imgbits   = $file_size['bits'];
    $imgchan   = $file_size['channels'];
@