I also put this as the wallpaper on my laptop which is equivalent to having the full-size version (or close enough). That has given me a chance to stare at it and thing about what it shows.
First, Aidan is certainly handsome, can't argue with that! But if he really were human, his expression shows arrogance and egotism at minimum and could indicate he is a sociopath. The look in his eyes is piercing and seems like he is sizing the viewer up to determine the viewer's suitability to be his next conquest. The smirk, instead of a real smile, helps amplify the idea he is arrogant, egotistical and maybe sociopathic. The fact he is a Cranshee who will bring death to his human "lover" for his own gain definitely makes him an evil entity. Did I say "sociopath?"
The way Nessa is standing with her head leaning against Aidan certainly indicates attraction but her expression shows melancholy and her eyes are pleading for someone to save her from what she knows is a fatal attraction but she also knows she cannot free herself. This is reinforced by the way Aidan's hands cover her hips, restraining her from walking away.
This is only my interpretation. Tami may have intended this or something else, I don't know. Others may see it quite differently. That's perfectly fine as we all view art through filters constructed from our life experiences. For myself, I've seen those expressions before (although not just on comic characters or television characters) since a major portion of my life put me in a position to deal with many couples preparing for marriage and I definitely have seen those expressions on some of the couples I dealt with. Almost without exception the couple either broke up before the wedding or divorced after very few years.
i downloaded a copy of the image and used The GIMP (open-source poor-man's answer to Photoshop) to scale it down to use as the wallpaper on my Android smart-phone (LG Optimus S). Really looks much better than any of the standard wallpapers!