Leona is the artist that has done a number of our covers, notably the Kati of Terra and Witches' Stones novels and the short story The Summer Cottage Mystery. Her partners in this were Sharon Fostermoore and Wanda Resek.
Here are some links to the work - to a photo album (12 pics) and a video (about 2 and a half minutes, with some nice soft music accompaning it).
Here's Leona's artwork on Kati 1, which I think is a smoking great cover, and lots of people seem to agree.
She also has work on Seeme and other art sites, such as Visual Arts of Alberta.