the image-upload field seem to store the image differently as the single-image field. the question is which function do i have to use instead of wp_get_attachment_image to fetch the image in the front-end
Yes, there is a difference when storing data between the field type image_upload and single_image. The field type image_upload has the settings 'multiple' => true which means the value returned is an array. You need to use the loop to iterate through array elements, like the cloneable field.