Further refine pointless splash image

The green plasma-like "smear" that shows up in a couple of frames may or may not be intentional (probably yes?), but it looked "off" to me, more like a rendering artifact than anything else. (I'm talking about this:)

So, I removed it. I didn't touch the halo around the symbol itself, or any of areas where the plasma effect rises upwards from it — just those bits off to the side.

YMMV, of course feel free not to merge this if you prefer it the way it is.

Along the way I optimized transparency on the GIF, which dropped the file size by 87%. I'd encourage doing that, regardless — in fact, I'll create a second PR with an optimization of the original, unedited GIF as an alternative, since optimizing a static-background image like that is a huge space-saver regardless.