Can I make my own image for my developer credit when the game opens or do I have to choose from defaults like in PS' RPG Maker 1?

Edit: Oh wait the RPGM1 thing was for the title page.

Everything in Ace is editable, so yes. The logos that appear before the main title are called "Logo1" and "Logo2" in the your game's system folder. You can just replace it with your own image (it must have the same name unless you alter what the name should be through the Ruby scripting).

Know that for good results the new picture should be the same size as the old. 544 x 416 are my files' sizes.

Excellent, thanks.