Importing Image for GUI

Hello everyone.
I want to import images for buttons and health bars etc.
What quality would be the best to make the images?
because i guess, too high resolution will require more memory
graphics etc. So what would the lowest quality
but good for pc game would be?

Thank you in advance!

If its for health bars and/or buttons. I would suggest about a 512x512 or less. Doesn’t take up to much memory but gets enough detail in to make it look good.