Not sure this would stand up in a court of law if the image was sent unsolicited and you promptly deleted it. Although deleted images in many cases can be recovered, and there may even be thumbnails of it you are unaware of (yes modern Windows versions create databases of thumbnails for every image you have saved), it could be argued that most people are ignorant to this and couldn't reasonably be expected to know that traces of the original file remain.
The image itself, you say it was risque but the girl was wearing underwear? You could legally view similiar images in swimwear/gymnastics catalogues if you were so inclined. I don't know what constitutes an indecent image these days, but it's definitely worth looking into. You need a good lawyer with an understanding of technology.
Also, I hope you saved or at least screenshot the blackmail attempt?