Yes, it is possible for a user to get the images in GPU memory. It's not likely they'd get them by dumping the entire GPU memory space (though with specialized hardware, drivers or debugging software such a thing is possible -- Visual Studio has a powerful GPU debugger these days). Rather, they can get them from CPU memory, which is much easier. Your game's entire address space is readable, plus they could install API hooks for D3D or OpenGL that intercept the texture upload calls to help figure out where the actual image data is.
Indeed, if somebody is particular unconcerned for the quality of the ripped textures, or your game has 2D ones, it's easy enough to simply extract them from screenshots.
You cannot protect against this from physically happening. As a general rule, anything in a game client is accessible to a user since they fully control the hardware it's running on.
You shouldn't worry about trying to protect your assets this way. Instead, rely on the fact that you are legally protected by intellectual property law.