The error message is quite clear, isn't it?
So for example your inner block should probably look like this (**args* fist, then ***kwargs*):
Div(
HTML("<p>Drag and drop photos and video here</p><p class='big'>+</p><p>Or click to add using the file browser</p>"),
css_id='image-drop',
css_class='dropzone span5 center'
)