Hey there, PNG is definitely better quality (lossless flat) whereas JPEG is lossy (compression artefacts)
I love that otherworldly landscape! I think using different shades of white to grey would help convey more depth to the globes in the background, nice job!
As for anchoring, I do in fact place my hand on the surface and let it slide on my hypothenar eminence wearing a drawing glove and when on the go, well no glove When I go for fine details I anchor onto the surface or the edge and hold the stylus closer to the tip (closer to the action means more control) and when drawing expressive broad strokes I change my grip - haven't completely decided on holding in classic draughtsman grip as I don't see much point to it in digital honestly but I grip much looser and sometimes from farther back the body of the stylus depending on the kind of stroke I'm going for. Basically experimenting. As for the origin of the strokes, I do make it a point to prop up my surface and use my shoulder as much as possible for art, wrist and elbow only when rendering and detailing
cheers