Right now that skull sits at around 550 triangles, which is the majority of your poly count. Because of viewing distances and so on, you can reduce the count of that skull by a helluva lot, and still maintain the same look when viewing the crate as a whole.
Probably around 250 triangles would do the job just fine, but I think (as a current employer, and a former teacher) you could use the practice making really low poly.
So here’s your challenge:
Duplicate the skull, move the duplicate to the right.
Reduce that duplicate to 250 triangles.
Duplicate the reduced skull, move to the right.
Reduce that duplicate to 100 triangles.
Duplicate the even more reduced skull, move to the right.
Reduce that duplicate to 60 triangles.
I think you’ll find that while viewing all of them at once, from the distance of framing the whole chest, that likely the 250 will look just as nice as your 550, and the 250 won’t really look that much worse.
If you are planning on normal maps, you can cut that detail way down. Remember that the most important part of a model that has normal maps is the silhoette. Things that bup inward can be entirely handled by the normal map - i.e. the eye and nose holes.