I try to get image url from image field.
I registered the field via php like this:
'key' => 'rectangular_image_field',
'label' => __('Rectangular image(optional)', 'mida'),
'name' => 'rectangular_image',
'type' => 'image',
'instructions' => __('The image will placed in the following sections: special project, main post, single post(bat the bottom of the excerpt)', 'mida'),
I understood from your docs that the default return is array, so I put this in the code to check:
Some fields may return the right thing even with an incorrect field key but that is only because many field types store text and in most cases ACF does not need to figure out what to do with the stored values. In the case of some fields, like image fields, ACF stores the attachment ID and needs the correct field key to find the field settings to see what you want it to return.