Фон изображений задается в файле ImageManager.php
// If image is a PNG and the output is PNG, fill with transparency. Else fill with white background.
if ($file_type == 'png' && $type == IMAGETYPE_PNG)
{
imagealphablending($dest_image, false);
imagesavealpha($dest_image, true);
$transparent = imagecolorallocatealpha($dest_image, 255, 255, 255, 127);
imagefilledrectangle($dest_image, 0, 0, $dst_width, $dst_height, $transparent);
}
else
{
$white = imagecolorallocate($dest_image, 255, 255, 255);
imagefilledrectangle ($dest_image, 0, 0, $dst_width, $dst_height, $white);
}