Well, the Sisters of Battle certainly look like they're enjoying themselves! Great concept art! But to be honest, I don't think the straight on angle works well: could you turn it more of a 3/4 view? I think that'd be even hotter!
Also, if you don't mind a little constructive criticism, you have Macha tied too tightly, she can't move at all: As a result, she looks too stiff. It would look SO much hotter if she had some leeway so she would look like she is struggling against her bonds a little!
One arm pulled stiff and the other partialy bent (but both wrists still tied.) as she struggles. Also you can do the same concept with the feet.
Another nice touch when doing a tickling picture, show some sweat beads on her skin. Matt some hair across her face as she laughs harder and harder!
Again you drew the sisters of battle perfectly! Maybe you could throw a few more articles of Eldar clothing on Macha so she looks a litle more like...well...Macha!
Apart from that, great job! Love to see what else you can do!