There exist tools that can connect to the graphics card and suck the rendered asset right out of video RAM. There even exist virtual machines that let you pretend to the program as if it's not being debugged, and as if time is progressing, even when it's actually stopped in a debugger.
Because the user needs to be able to see the pictures, it is possible for software to “extract” the pictures.
My advice is to not sweat the small stuff. It's five orders of magnitude more important to make a fun game, that people want to play, than to try to keep your sprites from being “stolen.”
First: Anyone stealing sprites is highly unlikely to actually make a fun game that people want to play.
And, second, if they somehow DO make a successful game, based on sprites that they stole from you, then you can trust the copyright law and legal system to actually get rewarded from that in the end. But that's … just very, very, unlikely to ever be needed. “Asset rips” for actual games being sold and successful, are super rare. Focus your energy on the risks you can control, like “is my game easy to find, easy to install, easy to play, and fun?”