Magento – how to upload an image for each store view

The quick and dirty way is to strip the protocol designator and turn it into a Protocol Relative URL:

// This assumes $product is from a collection that is constructed including a call to:
// Mage_Catalog_Model_Resource_Category_Collection::setProductStoreId()
$url = $this->helper('catalog/image')
    ->init($product, 'image')
    ->keepFrame(false)
    ->resize(250);
$url = preg_replace('#^https?://#, '//', $url);

Before trying anything, clear the cache. That might not help, but do it anyway.

The error is cause by this:

$this->_frontend = Mage::getModel($this->getFrontendModel())
            ->setAttribute($this);

this means Mage::getModel($this->getFrontendModel()) returns null (or false).
This can be caused by an attribute that has the field frontend_model set to something like [module]/something_here but the class [Namespace]_[Module]_Model_Something_Here does not exist.
You can identify the attribute by adding the following code above the line I mentioned:

$model = Mage::getModel($this->getFrontendModel());
if (!$model) {
    echo $this->getAttributeCode();exit;
}

You should see the attribute code in your page.
Then look in the eav_attribute table for this attribute and see if something is wrong with it. If you think the attribute is not needed anymore just delete it.