Merhabalar @codepacks. İlginiz için teşekkür ederim. Yaptığınız kod çalışıyor fakat resmin quality'sini %100 yapmama rağmen görünmeyecek halde bozuyor.
Aldığım çıktı şu şekilde: https://prnt.sc/35C8KU80fumL
Resmin orjinali: https://prnt.sc/2gytP5_LmFU4

Ayrıca Dosayı küçültünce çıkan hatam:

    Warning: Undefined variable $file_ext in C:\xampp\htdocs\photo\upload.php on line 34
    
    Warning: Undefined variable $file_ext in C:\xampp\htdocs\photo\upload.php on line 37
    
    Warning: Undefined variable $file_ext in C:\xampp\htdocs\photo\upload.php on line 40
    
    Warning: Undefined variable $file_ext in C:\xampp\htdocs\photo\upload.php on line 43
    
    Warning: Undefined variable $file_ext in C:\xampp\htdocs\photo\upload.php on line 50
    
    Warning: Undefined variable $file_ext in C:\xampp\htdocs\photo\upload.php on line 54
    
    Warning: Undefined variable $file_ext in C:\xampp\htdocs\photo\upload.php on line <57></57>

KODUM:

   <?php

$file_name= str_replace(" ","-",$_FILES['resim']['name']);
$resim=rand(0,99999)."_".$file_name;
//orjinal halinde kayıt yeri (images/)
$tpath1='images/'.$resim;
$pic1=compress_image($_FILES["resim"]["tmp_name"], $tpath1, 90);

//kırpılmış halinde kayıt yeri
$thumbpath='images/small/'.$resim;
$thumb_pic1=create_thumb_image($tpath1,$thumbpath,'470','260'); // kaydedilecek resim boyutu değiştirilebilir
$file_name= str_replace(" ","-",$_FILES['resim']['name']);
$resim_cover=rand(0,99999)."_".$file_name;

function compress_image($source_url, $destination_url, $quality) {
    $info = getimagesize($source_url);
    if ($info['mime'] == 'image/jpeg')
        $image = imagecreatefromjpeg($source_url);
    elseif ($info['mime'] == 'image/gif')
        $image = imagecreatefromgif($source_url);
    elseif ($info['mime'] == 'image/png')
        $image = imagecreatefrompng($source_url);
    imagejpeg($image, $destination_url, $quality);
    return $destination_url;
}
function create_thumb_image($target_folder ='',$thumb_folder = '', $thumb_width = '',$thumb_height = ''){
    $target_path = $target_folder;
    $thumb_path = $thumb_folder;
    $thumbnail = $thumb_path;
    $upload_image = $target_path;
    list($width,$height) = getimagesize($upload_image);
    $thumb_create = imagecreatetruecolor($thumb_width,$thumb_height);
    switch($file_ext){
        case 'jpg':
            $source = imagecreatefromjpeg($upload_image);
            break;
        case 'jpeg':
            $source = imagecreatefromjpeg($upload_image);
            break;
        case 'png':
            $source = imagecreatefrompng($upload_image);
            break;
        case 'gif':
            $source = imagecreatefromgif($upload_image);
            break;
        default:
            $source = imagecreatefromjpeg($upload_image);
    }
    imagecopyresized($thumb_create, $source, 0, 0, 0, 0, $thumb_width, $thumb_height, $width,$height);
    switch($file_ext){
        case 'jpg' || 'jpeg':
            imagejpeg($thumb_create,$thumbnail,100);
            break;
        case 'png':
            imagepng($thumb_create,$thumbnail,100);
            break;
        case 'gif':
            imagegif($thumb_create,$thumbnail,100);
            break;
        default:
            imagejpeg($thumb_create,$thumbnail,100);
    }
}

Bu konuda ne yapabilirim?