A very simple method to create a watermark that works decently well is to use the free software, "Gimp." It's very much like photoshop, without some of the bells and whistles.
You create a new file, and tell it to make the background transparent. Then use the text took, type some simple text in whatever font you like. (GiGi would be yours I assume.) The go to "filters," "light and shadow" "xach effect." Add the xach effect to your text, which creates a highlight later and a drop shadow layer.
After you've added this, re-select your text layer and delete the text. You'll be left with a highlight, a drop shadow, and a transparency.
Merge all visible layers, and then use the select tool to copy/crop the area you want for your water mark, then past this selection as a new file. Save it as "mywatermark" or something of the sort (save it as a .png file if you're going to upload it to zenfolio).
Once you've created your .png watermark file, you should be able to use the options in zenfolio to upload it and set it to automatically lay over any images you upload to albums set to use that watermark. (The file name will now show up in the list of watermark options in each zenfolio album.)
As for making your images look more professional, I don't think that is what you meant, but it did originally sound that way to me =P A watermark, to me, is nice because it's like a way for me to add my signature to my art. It doesn't really add or detract from it (unless the watermark is too obnoxious...) but it lets the viewer know who created the picture they're enjoying. I like watermarks in most situations, but I wouldn't want to watermark anything unless I really was proud of it =P
Good luck.. If any of this was confusing there are lots of tutorials out there, as well as good youtube videos!