$src_img=imagecreatefrompng($name); //Loads the source image's name and location, either from a variable or (preferably) an array
$old_x=imageSX($src_img); //Get's the source image's with
$old_y=imageSY($src_img); //Get's the source image's height
$dst_img=ImageCreateTrueColor(170,40); //Creates a canvas for the thumbnail, 170x40.
imagecopyresampled($dst_img,$src_img,0,0,0,0,170,40,$old_x,$old_y); //Resizes the source image to 170x40 and then copies it to the blank thumbnail canvas
imagepng($dst_img,$thumbsfolder."/".$filename); //Saves the new thumbnail to whatever folder/filename you want.
imagedestroy($dst_img); //Deletes the thumbnail image from ram, as it's no longer needed.
imagedestroy($src_img); //Deletes the source image from ram, as it's no longer needed.