I tend to agree with @AnnaLandin, but I would like to add something else to the mix. I feel that promo art is just as much about what isn't revealed as it is about what is shown to the audience. Having characters interact with each other is a great way to get their individual personalities across, which is very good for character based comics. Alternatively, an illustration where something doesn't seem right, or where something seems out of place can do wonders for piquing curiosity.
For example, I'm currently working a promo illustration for the main storyline of Kolodruidtale. (Note: This isn't the epilogues series that is currently available on the site). In this illustration, I have two of my main characters with their backs to the camera, looking out into the distance. They are dressed very well, almost in full ball gown attire.
In the distance, however, the camera sees a large trail of destruction, cutting a wide swath through an otherwise very prosperous city. Through the carnage, we see a peaceful vista of the distant shores(The city is by a large body of water, a Gulf).
It's not yet finished, but the intent is to raise questions just as much as I give real information about the storyline. Then again, text might not quite do it justice, but I hope I have explained myself well enough. 
By the way, @keii4ii, I'm a die-hard fan of your work, Heart of Keol. The stunning quality with which you publish each page both inspires and motivates me. @AnnaLandin, the same goes for you, as well. I'm working hard day and night, all in pursuit of someday being as good as you guys. 