Hi!
I am struggling to get a DataObject working with multiple image-upload.
Its working with a has_one relation as the code shows.
I can't find out the correct relations when trying to set this up with a has_many images relation. ( in MyImage )
Thanks! timo.

ProjectPage:

class ProjectPage extends Page {
	
	static $has_many = array(
		'ProjectItems' => 'ProjectItem'
	);
	
	static $description = "ProjektSeite";
	static $singular_name = 'Projekt';
	
	public function getCMSFields()
	{
		$fields = parent::getCMSFields();
		$fields->removeFieldFromTab("Root.Main","Content");
		
		$GridFieldConfig = GridFieldConfig_RecordEditor::create();
		
		$GridFieldConfig->removeComponentsByType('GridFieldPaginator');
		$GridFieldConfig->addComponent(new GridFieldPaginator(20));
		$GridFieldConfig->addComponent(new GridFieldSortableRows('SortOrder'));
		
		$Gridfield = new GridField("ProjectItems", "Projekte", $this->ProjectItems()->sort("SortOrder"), $GridFieldConfig); 
		$fields->addFieldToTab('Root.'.$this->Title, $Gridfield);
		
		return $fields;
	}

ProjectItem:

class ProjectItem extends DataObject
{
	static $db = array (
		'SortOrder' => 'Int',
		'Title' => 'Varchar(255)',
		'ShortDescription' => 'Text',
		'Description' => 'Text',
		'ExternalLink' => 'Varchar(255)',
		'ServiceText' => 'Text'
	);
	
	static $has_one = array (
		'ProjectPage' => 'ProjectPage',
		'Thumbnail' => 'Image'//,
		//'Images' => 'MyImage'
	);
	
	static $has_many = array (
		'Images' => 'MyImage'
	);
	
	/*public static $many_many = array(
		'Images' => 'MyImage'
	);*/
	
	static $summary_fields = array(
		'Thumb' => 'Thumbnail',  
		'Title' => 'Title'
	);
	
	static $singular_name = 'Projekt';
	static $plural_name = 'Projekte';
	
	public function getCMSFields()
	{
		
		$fields = parent::getCMSFields();
		
		
		$fields->removeFieldFromTab("Root.Main","SortOrder");
		$fields->addFieldToTab("Root.Main", new TextField('Title', 'ProjektName'));
		$fields->addFieldToTab("Root.Main", new TextareaField('ShortDescription', 'KurzText'));
		$fields->addFieldToTab("Root.Main", new TextareaField('Description', 'Text'));
		$fields->addFieldToTab("Root.Main", new TextField('ExternalLink', 'Link'));
		$fields->addFieldToTab("Root.Main", new TextareaField('ServiceText', 'Leistungen'));
		$fields->addFieldToTab("Root.Main", new UploadField('Thumbnail', 'Thumbnail'));
		
		
		$myImageGridFieldConfig = GridFieldConfig_RecordEditor::create(); 
		$myImageGridFieldConfig->addComponent(new GridFieldBulkManager());
		$myImageGridFieldConfig->removeComponentsByType('GridFieldPaginator');
		$myImageGridFieldConfig->addComponent(new GridFieldPaginator(20));
		$myImageGridFieldConfig->addComponent(new GridFieldSortableRows('SortOrder'));
		
		$fields->addFieldToTab("Root.Images", new GridField("Images", "Bilder", $this->Images()->sort("SortOrder"), $myImageGridFieldConfig)); 
		
		
		
		
		return $fields;
	}

MyImage:

class MyImage extends DataObject {
	
	static $db = array (
		'SortOrder' => 'Int'
	);
	
	static $has_one = array ( 
		'Image' => 'Image',
		'ProjectItem' => 'ProjectItem' 
	);
	
	/*static $has_many = array ( 
		'Images' => 'Image'
	);*/
		
	/*public static $belongs_many_many = array(
		'ProjectItem' => 'ProjectItem'
	);*/	
	
	static $singular_name = 'Bild';
	static $plural_name = 'Bilder';
	
	public function getCMSFields(){ 
  		$fields = parent::getCMSFields();

		$fields = new FieldList(
			new UploadField('Image','Photo')
		);
		
		return $fields;
	}

I'm not sure what you are actually trying to accomplish, and what the problem is. But I know it's best not to use a has_many relation to an Image object - use a many_many relation instead. I've had it working nicely together with the SortableFile module:

http://addons.silverstripe.org/add-ons/bummzack/sortablefile