I decided to find out what would happen if you had an image file larger than normal. I tested it with a diamond pickaxe, making it longer on the handle and the edges to fit a 64x64 space using poor image editing and I went to test it in-game.
But before I could even get to test it, the game wouldn't even load the title screen.
"Minecraft 1.14.1 (Not Responding)"
"Minecraft 1.14.2 Pre-Release 1 (Not Responding)"
"Minecraft 1.14.2 Pre-Release 2 (Not Responding)"
That's what it said on all the windows.
After leaving one of them open for an hour, it still never loaded.
I eventually deleted the entire folder, along with the item file that caused it and tried reloading the game and the same issue occurred, even with the files erased.