Here is how I get my image:

$coverurl  = 'https://api.someurl/api/v1/img/' . $somenumber . '/l';
//$iheaders contains: 'Content-type' => 'image/jpeg'
$iresponse = wp_remote_get($coverurl, $iheaders);
$img = $iresponse['body'];
$testimg = base64_encode($img);
When I echo $testimg with an img-tag, everything works fine.
echo '<img class="attachment-shop_single size-shop_single wp-post-image" src="data:image/jpeg;base64,'.$testimg.'" width="274" />';
Since I need to convert the string into a jpg and save it to my uploads folder, I tried to use imagecreatefromstring().
$imgx = imagecreatefromstring($testimg);
        if ($imgx !== false) {
            header('Content-Type: image/jpeg');
            imagejpeg($imgx);
            imagedestroy($imgx);
        } else {
            echo 'An error occurred.';
But I never get to the point of saving anything, because of the following warning:
 Warning: imagecreatefromstring(): Data is not in a recognized format in /etc.
When I echo $testimg I get:
/9j/4AAQSkZJRgABAQAAAQABAAD ...Many number and characters.. Ggm2JUsEvfqnxAhGFDP/9k=
What must I do to make createimagefromstring work? Do I have to modify the $testimg string? Thanks for your interest.
The method imagecreatefromstring does not take a base_64 encoded string. Try this instead:
$imgx = imagecreatefromstring($img); // Contents of $iresponse['body']
You can see this in the topmost comment in the documentation page (linked above):
$data = 'iVBORw0KGgoAAAANSUhEUgAAABwAAAASCAMAAAB/2U7WAAAABl'
       . 'BMVEUAAAD///+l2Z/dAAAASUlEQVR4XqWQUQoAIAxC2/0vXZDr'
       . 'EX4IJTRkb7lobNUStXsB0jIXIAMSsQnWlsV+wULF4Avk9fLq2r'
       . '8a5HSE35Q3eO2XP1A1wQkZSgETvDtKdQAAAABJRU5ErkJggg==';
$data = base64_decode($data);
$im = imagecreatefromstring($data);
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Do the following manipulations on the image:
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