No, because this will not be bound to the object in the situation you show.
I don't think there is a way to do this.
You could try
var gallerySelectors = {
'$thumbnailContainer' : $('#thumb'),
'thumbnailImg' : gallerySelectors.$thumbnailContainer.find('img')
}; ^--------------- NOTE!!!
but I'm not sure whether I would trust this to work everywhere. I guess you'd have to look into the language spec to see whether the members are always guaranteed to be initialized in order.
I would just take the easy route:
var gallerySelectors = {
'$thumbnailContainer' : $('#thumb'),
'thumbnailImg' : $('#thumb').find('img')
};