i suppose you are using vbgallery 2.5
in that case (same with 3.0)
place a watermark image in your (example) gallery/files directory
then in ACP ->vbgallery -> modify settings, in section upload
find Watermark Images
there you enter the full path (not the URL!!!) to the file..
you will then have a non-permanent watermark on your images..
if you want a permanent watermark, you will have additionally to set Permanently Watermark Images to yes ...
this will only work on fresh uploads..
if you want to permanently watermark existing images, you will have to, (after you did the settings) goto ACP->vbgallery->maintenance there select permanently watermark images.
but attention, this cannot be undone. (so I would test watermark with non permanent, and only set it to permanent once you are satisfied.)
To remove non-permanent watermark, just remove the entry in Watermark Images and leave it blank.