There is no known portrait that has ever been positively identified as Kathryn Howard. It is possible that all paintings (if any were made) were destroyed after Kathryn's execution and little other mementos of her short time as Queen survive to this day.
The above miniature was identified as Kathryn Howard in the 1840s, but that has never been confirmed. Some reserachers now think that this may in fact be Kathryn, based on the fact that she is wearing jewelry that had belonged to Jane Seymour. (see below) The jewelry would have been part of the 'Queen's Jewels' which means that after Jane's death, they probably would have gone back to the king and then passed on to his future wives. [One problem to this theory is that the colors of the stones don't quite appear to be the same in each painting. However, one was a miniature, and one was a full-sized portrait, which could perhaps account for some of the variations. Plus the over all color of "light" in the paintings are quite different.]
Susan James has published an article proposing that the miniature is of Margaret Douglas, Countess of Lennox (see: James, Susan E. "Lady Margaret Douglas and Sir Thomas Seymour by Holbein: Two Miniatures Re-identified" Apollo, Vol. 147, No. 435, 1998).