It depends which setting you have in the Gantry 5 system plugin, see attachment.
Easiest way to find an image path, on frontend, right click on the image > Open image in new tab or Show image info, etc
OK found it in template/custom/images, but that's not where other images are stored.
Gantry-media stream seems to store particle images in random locations, and it's hard to track them down. Can someone explain the algorithm of when it stores in rt_template/images, vs rt_template/custom/images, vs public_html/images, vs gantry5/assets/images, etc.?
Seems weirdly random, but when you select them in a particle within gantry-media, they all appear to be in one place. Very confusing.