Solution possibly found (Patreon)
Content
To solve this issue, I ran all the splash images in the game through a compressor. I checked each image to make sure none of them compressed poorly. You shouldn't be able to tell a difference on most images, and a few images only have a small amount of visible dithering.
The plus side to this fix is that the game's filesize went down from 123mb to 59mb. Which means this will also fix the newgrounds filesize limit problem and allow me to embed the game into the blog again.
I can't say yet for 100% certain if this fix will hold, so I will be working on learning another engine in my spare time just in case, but for now I am able to add images to the game once again.