[Disclaimer: I make no warranties nor guarantees 
as to the quality and ability of these graphical 
enhancements in preventing/thwarting thieves who 
are truly intent on using your images... Thus is 
the nature of the internet. -_O]

"Watermarks" in general are transparent or faded 
out over an image. So technically when I'm talking
about opaque ones they're not really watermarks...

I'd say transparent watermarks may be removed by 
individuals with intermediate-high knowledge of 
a high-end graphics application (e.g. Photoshop).

Opaque 'brands', then, would require much more 
labor to remove considering:

1) They are not placed on a solid background. 
Anything placed on a solid background can be 
merely painted over.

2) They are placed over an area of detail. Hair, 
detailed clothing, hands, etc., especially 
detailed /backgrounds/. Marcus' Slayers bg'ed cel
("Lack of self control..." thread) is a great 
example of an opaque brand over a detailed bg - 
extremely difficult or impossible to remove.

3) They obliterate or obscure lines. Lines are 
important - if you can place an opaque brand over
a group of lines (i.e. detail), this makes it 
very labor intensive to extrapolate them. Or even
a single, long line, like in hair.

4) They obscure color gradients or the points 
where one color changes into another. 

I know the opaque brands may look ugly, but you 
can consider matching colors slightly (not too 
closely!) so they don't stand out too much. 
Placement is everything... x_x  Thasallfornow.