Classically, Ereshkigal is the daughter of Anu (Grandfather Sky) and Nammu (Grandmother Ocean), the twin of Enki, the Lord of Earth, the master-craftsman, god of learning, civilization, and magic. Like almost all mythological twins, they (to some extent) represent light and dark, day and night, knowledge and wisdom. Although she is often referred to as Ishtar's sister, I understand her to actually be her godmother/initiator.
No known portrayals of Ereshkigal survive, although some people associate her with the "Queen of Night" relief.
There is one image that I associate with Ereshkigal. I wish I was a painter, so that I could better convey it to you. While it had been sort of swirling about in my head since the night I met Ereshkigal, it wasn't until quite recently that it began to crystallize. Vision is not my strongest sense; I am far more likely to hear a spirit than to see it, most of the time. In any case, it was when I saw this (fragment of a) figurine, recovered from the neolithic site at Catalhoyuk, Turkey that my vision snapped into focus.
In my vision, a great goddess, dark skinned, very fat, possibly pregnant, pendulous breasted, her hair dark, thick, tangled, possibly in dreads, the ends glowing like embers. She is spinning. With each rotation, she is consumed in flame, and her flesh falls away, only to remerge as she continues to spin. Behind her, is are seven wide, black stairs up to the dais of her throne, shining onyx, snakes entwined on the arms.
I realize, as I write this, that while I told you I first met Ereshkigal last winter, that's not true at all. I met her when I was about 12, in the Tombs of Atuan with Tenar.
If you'd like to journey to meet her, here is an audio recording I've made to help facilitate it. https://drive.google.com/file/d/0B_upeofrIpFJb2RKaVBfaFQ1WEU/view?usp=sharing
WARNING: The Underworld is not to fuck around with. Particularly if you don't have experience with trance work, you should take this seriously. I've already gotten one "What the fuck did I get myself into." email.