This is a good question and on-topic as far as I'm concerned. As you correctly point out our furthest reaching spacecraft is only at the heliopause, which is no real distance astronomically, and certainly not far enough to get a glimpse of our galaxy from an angle and distance which would allow that self-portrait. The images of the milky way galaxy are all artistic renditions, they are educated guesses of what our galaxy would look like based on observations of other galaxies and our own. Concentrations of matter have been extensively mapped in our galaxy using visible light and spectrum outside the visible range like infrared, x-rays, and others. These concentrations of matter show the structure of our galaxy even though we cannot observe it from an angle.
The next logical question is how do we know they are right, and the answer is we don't, not with absolute certainty. The map of our galaxy is based on observations from a single point, and interpreted using an understanding of the physical laws of our universe as developed over hundreds (or thousands) of years by some of the finest minds in history. Yet for all of the effort and brainpower it is still within the realm of possibility that this understanding is wrong in which case our map may be wrong.
It's going to take a long time before anyone gets far enough away to check that work in any case.