For illustration purposes, I ran the second picture in lab mode, extracted the lightness channel and applied a levels adjustment to it (reducing darkness and increasing the midtones). The image below was the result. Fairly easy to OCR. Now, my approach may not even be the easiest. But I think that by using lab mode, I guarantee I can tackle for any kind of noise in the image. As far as coding is concerned, I suppose this could be fairly easily achieved by someone bent on defeating this captcha.