Hard to say; while her outfit in the other picture is closer to what we normally see on Maribel, Hammer has drawn Yukari wearing that (and with purple eyes) before.
We only have twopictures of Maribel by Hammer, so there's not a great deal to go on - but given the lack of Renko, and the fact that the other pic still had ribbons on the cap (which I don't think Maribel has ever been shown with), I'd have to say it's still Yukari both times.
This one looks more like the outfit we see Yukari wearing in IaMP, while the other one is more like her appearance in PCB. They kind of do give off different moods, since the IaMP outfit tends to cover more and make Yukari seem less... vulnerable, I suppose? (Though that's more apparent in a full-body image)
