A couple of days ago April pointed out that someone could get around the download credit system by right clicking on an image and selecting "save image as."
Now for most of the images on Pixel Scrapper that's not really much of an issue, because the full-size images you get when downloading are much, much larger / higher resolution than the large thumbnail you get when you look at the item on the site; the large thumbnail isn't really worth a whole lot. But still, the large thumbnails are large enough that some people might think of using them for things like embellishments, instead of actually downloading the full-resolution images.
The watermarks might solve this problem, but they're also rather ugly. We don't want to punish everyone with ugly thumbnail images, just because a few people might abuse the system... so we're thinking of getting rid of the watermarks.