well awesome just showing panties is never sexy when they are just thrown in your face ;D.
atleast i always find things sexy in art when its done really subtle and flow nice into the figure.
here like this artist has some sexy a** art http://plusnine.deviantart.com/art/15082009-133440359 like even tough her a** is thrown in your face her panties really flow into the figure and are done somewhat subtle
my fav pic of him is this one http://plusnine.deviantart.com/art/Trey-01-104303642 the one to the right is just perfect !!
so yah what you can do is might add some left over strings of thread over her leg covering up her panties more. making her breast cleavage pop out more might work too. More skin is always big points ;D
but the biggest problem is probably her expression is to serious.
---- anyways this what i had in mind, but it will be alot of repainting for you xD