I had this issue too and I finally found how to fix it here http://www.nabble.com/Binding-an-arraycollection-of-Loaders-td14756754.html.
This is what to do:
<mx:Image x="10" y="10">
<mx:source>
<display:Bitmap
bitmapData="{data.thumbnailLoader.content.bitmapData}">
</display:Bitmap>
</mx:source>
</mx:Image>

