Jehovah as a bull
Jehovah was actually traditionally viewed as a bull god even back in the times of the Israelites. This article -Worship of Yahweh as a Bull- goes into the reasons of why the Isrealites saw Jehovah as a bull and what evidence in the Bible supports it. I'll paraphrase in case you don't feel like reading that article:
-The Caananite god Baal was a bull god and the Israelites lived next to them for a long time. The Israelites kind of took the idea of a bull god from them but declared Yahweh to be their superior version. (That's just one theory)
-Numbers 24:8 says that god "has the strength of a unicorn" in the KJV. Other versions render this as "the horns of a wild ox."
-1 Kings 12:28-29 The Isrealites set up two "calves of gold" in Beth-el and Dan. They call them the gods that brought them out of Egypt (making the idols a representation of Jehovah). Jeroboam actually did this to represent Jehovah because he didn't want the ten tribes returning to Jerusalem for worship, as the kingdoms had just split and he wanted them to have no unnecessary interaction with Judah.
-Exodus 32:3-4 Aaron makes the Isrealites a molten calf to worship and the people said "These be thy gods, O Israel, which brought thee up out of the land of Egypt." (again a reference to the molten calf representing Jehovah's role)
-Marduk, the chief god of the Babylonians (who conquered the Israelites), is known as the young sun bull.
I think Hades being represented by a skull is pretty obvious as he's the lord of Sheol / Hell. People go there when they die. Hence the skull. I don't have an article for that because it's more in the realm of common knowledge. Feel free to let me know what you think of the cover so far or comment on anything else relevant to the topic.
Jehovah and Hades is available on Amazon. Check it out here.