Best program to work in/format to save art as when creating 2D assets or concept art?

Is there a preferred way to create and save digital 2D art for games? I'm relatively confident in my actual art skills, but I've never actually worked in a game development team before, despite wanting to for a really long time, so I don't really know proper “game developer etiquette” (if there is such a thing lol). I can guess that most professional 2D concept artists or asset creators probably use Photoshop, and I am quite comfortable with that program, but right now, most of the time I just use IbisPaintX despite the ads, just because it's free and I'm broke. Is there a better program that I should be using? What would you guys recommend?

Thanks,

maeve