If you're wanting to watermark your pictures using company logos or even your own signature then the easiest way is to make a custom brush. Think of a brush like a stamp cut out of potato and dipped in paint! Brushes can only be one color (black)
Scanning your signature, handwriting or logo
First write clearly on a white piece of paper using a black pen. Next you need to scan it so you have it in Photoshop. You might want to scan at 200% or even 300% to increase the size of the scan - bigger the better! Once you have your signature in Photoshop we need to remove as much noise as possible which will result in a crisp & clear image. Do this by adjusting the levels.
Layers>New Adjustment Layer>Levels (CTRL+L)
http://www.pictureinthesky.net/external/wpf/levels.gif
Note how I've moved the black setting in from the left and I've also moved the white setting in from the right. Once you're happy with the new levels click "OK" to apply. If you can see any obvious blotches that shouldn't be there now is the time to remove them using the Eraser tool (E).
If you've scanned a company logo you can play with the levels to make it as black as possible without losing too much detail. If it's in color convert it to B&W by "Layers>New Adjustment Layer>Channel Mixer" and ticking the box called "Monochrome".
Once finished it's safe to flatten everything (Layers>Flatten Image).
http://www.pictureinthesky.net/external/wpf/example.gif
Now to convert it into a brush. From the "Edit" menu select "Define Brush Preset..." and you'll get something like this:
http://www.pictureinthesky.net/exter...efinebrush.gif
Give it a name so you can easily recognise it and click "OK". Any white space around the image will be removed automatically.
Using your brush
Before you apply your brush to your picture it's best to make a new layer. This way you can drag your brush around the picture for best placement as well as applying any blending options to make it stand out like adding an outline. You can change the size of the brush by using the size slider (green, below pic) and this is why we need a very big scan - if it's too small and you increase the size of it then it will start to pixelate.
Saving your brush
http://www.pictureinthesky.net/exter.../brushmenu.gif
With the brush tool selected open the menu (blue) and clicking the small arrow (red) and selecting "Save Brushes".
This will save all the brushes currently selected so you might want to remove all those preset brushes first. Instead of selecting "Save Brushes" select "Preset Manager" instead.