Custom field image : front, OK – Back : out…

Hi,
nice plugin, but a weird thing is going on on my web site…
I added an image custom field in the buddypress profile.
When I wanted to add a picture from the backoffice (WP user profile), nothing seems to happens : the field stay without picture, and without any file path. In front office, nothing also is displayed.
But if I manage the profile in front office, the image is uploaded and is displayed in the profile.
What could be the problem ?

Thanks.
Laurent