Adding a watermark to several images with a different horizontal and vertical aspect ratio and resizing them at the same time may seem to be impossible.  There are however a few not so known functions in Photoshop to help you to automate the whole process.

Since most of the original photos or pictures are larger in size than the size a normal website layout can accomodate it's better to resize them to a common maximum width or height for a gallery or articles.  The first tool most people would think of is the image resize command in photoshop.  The disadvantage with that command however is that the maximum allowed size can only be either width or height.

Placement of a watermark in the lower right corner can be a problem as well .  Actions record transformations relative to present location.  Again an image with another aspect ratio will not work with that action, requiring manual and tedious sorting before applying a prerecorded action for each case.

prepare :

• make a psd with your watermark and save it in a sensible place
• load up an example image
• make sure the image has at least 2 layers
• prepare to record the action

The action itself :

• flatten the image
• use the fit image command in the file->automation menu
• file->place the premade logo (make sure the pixels aren't all transparent or the next few commands won't work)
• select all
• allign layer to selection right
• alligh layer to selection bottom
• transform and change the position of the watermark layer to add a margin (use the arrow keys)
• change the transparency of the layer if needed / add layer effects
• flatten image
• done

optionally you can define the way the image should be saved, but that can be done in the batch process requestor as well.

Hope this helps you all along