68kMLA

You have to edit the icon in the System file using ResEdit.

Always work on a copy of the System file, not the original.

larger graphic will be an issue. the welcome to macintosh box is drawn by the system error handler, so you have limited customisation options. easiest option would be to write a little INIT that just doodles over the top of it and load it early but you'd still get a picasso flash.

Missed where you said "larger". I'd just fake it with a startupscreen file that looked like the dialog box.

You could always just blank out the picasso Mac icon to prevent a "flash". In 7.1.1 I don't think the picasso Mac is used anywhere else.
I did try to achieve this effect using a StartupScreen but it doesn't appear in the same spot as the old dialog and on my LC it loads slowly making it look super fake. Modding the original dialog is the most efficent option.

then an INIT early in the boot sequence is your best bet.

how would I write that?

Have you written any Mac software before? Do you know C?

I’ve never written classic Mac software before but I’ve messed around with C++ a bit and to a larger extent, easier languages like Python and Java

OK. I might suggest first getting a beginners' mac programming book - I don't know which ones are good, I just hit my head on things until they worked, which I don't recommend - and learning how QuickDraw works, which is the graphics API...

... After that, writing a simple INIT is pretty easy - there's plenty of code samples out there - and that's just a blob of code that runs at boot time and you can just doodle on the screen using QuickDraw then.