I don't think these are really mistakes per se. Most images on set packaging are either CG models, photoshoped or photoshoped CG models.
If you start to examine box art in detail, you'll find sets bending in ways they normally don't. Look at Gresh picture again. His chest is slightly bent. It's not a flaw; it's so the image looks more dynamic. If you want better examples, look at some of the 01 art.
As for the skull spider, yes, that is a flaw. The background art must have been before the change was made. Sometimes changes in sets are very late in the game and other areas don't get the memo in time.