Ok dumb question, so dumb answers appreciated..
Why is the Milky Way and for that matter the plane of the ecliptic curved in most images of the Milky Way/sky?
Is it a lens thing or is it some kind of atmospheric effect or the sun''s position off the galactic equator? Or any combination of the above.
Dons last picture does not demonstrate this effect but basketballfreaks6's does in spades.
The Milky Way always looks curved to the eye. The earth is a sphere and from our point of view on the earth, the Milky Way is a circle around that sphere. We never see the entire circle, since some part of it gets blocked by the earth we're standing on. So we see a half circle.
The seemingly straight and curved images come from differences in the field of view and potentially some distortion from the imaging process. The larger the section of the milky way, the more apparent the curve will become in images.
If you look closely, the two images you used as examples contain different sections of the milky way - the curved one has the full half cirlce. The other one just appears straight because the section of the milly way shown is too short to perceive the curve.
In addition, many images of the night sky are compesed of multiple shots. Panorama stitching software may introduce some distortion to deal with the extreme perspectives.
Perspective can get hard to imagine based on words alone. I suggest you try some ultra wide shots of the milky way yourself if you want to get a feel for it.
Edit: I guess another way to think about it: If something curves around you, meaning it is to your left, right and above you, how would you capture that in a 2D image? The left and right part is easy, just put it left and right of the images center (direction you're looking in). The "above" part is conveyed through putting it above the center in the image's 2D plane - but that automatically requires the thing to appear curved.