Ok, I played Radiant. Here are my thoughts about this:
Your graphics (and type of game)
I'm sorry, but except being a shooter genre, the gameplay is completely different. There is no spaceship in DI and thus no movement around, there are no bullets, boss levels are completely different, etc. If you played both games, you would easily find Radiant more similar to other games than DI.
are extremely similar to the game Radiant's
I looked at Radiant graphics, and I can only find two aliens that are similar (the minions on boss levels in wave 10 and 20). I have to admit they are somewhat similar, but nobody can claim I lifted the aliens from there as there are clear differences. Besides, I've seen such aliens in many games before. If you start from Space Invaders onwards, I bet you'll find more than one game that has both of these alien types. It's just a coincidence that Radiant is a newer, popular title, and thus more people know about it.
Even if you did make those graphics yourself, it'd be best to change them, or people will accuse you of copying.
Hm, maybe you're right. I just found a thread on Reddit and one of commenters also claimed the same. I'm seriously thinking to change those, but I'm not sure if this is even a good thing to do, because:
1. it would be admitting I used aliens from Radiant, which I did not
2. someone would probably find many other aliens similar to some other game, so I could be redrawing graphics all the time.
What would you guys do in such position?