It's been said you shouldn't try to simplify things before you actually make anything because in the end you'll make things worse, but that's also probably why so many games these days waste resources. In many cases you can tile something that normally wouldn't be thought of as tiled. Take the image in question and apply an 8x8 grid over it. Look for ANY tiles that are duplicated. Don't see any? Consider reducing the color count or changing some tiles so they're duplicated. If that's not an option and you can't break it down, then go ahead and use the full image.
And games these days ARE wasteful. Programmers don't take the time they need to to optimize their space and coding. Everyone nowadays has the destructive mentality that since the media can hold larger ROMs, there's no need to optimize the ROMs. For sprite rippers, it's nice because it's easier to align sprites, but moving from ripping NES and SNES sprites to ripping GBA and DS sprites, it's become apparent how wasteful designers have gotten. In GeGeGe Kitarou on the GBA, if the programmers optimized their sprites, they could have freed up possibly 32kb or more. That might not sound like much, but in terms of graphics memory that is a lot.