Might try here too 
https://www.hard-light.net/forums/index.php?topic=94951.0
Oh, very nice. I've been looking to upgrade a couple of skyboxes as well, so this might help.
Oooh SS2 mods.
Funnily enough, I was looking at SS2 mods this weekend. I'm getting the urge to play it again, the Metal Mother calls methinks 
Hopefully you were looking on Systemshock.org and not ModDb.
Anyway, this is a bit embarrassing, but right after posting this topic I went on another dive into stock footage land and actually found the two explosions I was looking for!
https://www.pond5.com/stock-footage/item/86915050-explosion-flaming-fragmentshttps://www.pond5.com/stock-footage/item/86915078-vertical-explosion-cloud-sparkling-debrisI do have a related question, and hopefully some of the FSO coders see this. It has to do with actually rendering the explosion bitmaps. The original explosions were tiny 64x64 bitmaps with 1-bit alpha, rendered with no blending. The resolution and frame rate were so low that it was practical for the artists to hand-tweak the transparent regions. This obviously isn't practical for animations running at quadruple the resolution and frames. So far I've tried...
Use the magic wand tool on each frame to create the alpha channel. This results in a black fringe around each explosion frame. Tweaking the sensitivity of the wand to reduce the fringe ends up removing darker parts of the actual explosion.
Use the luminance as the alpha. This doesn't look terrible, but gives an odd almost photo-negative effect when viewed against a light background, since the internal dark regions become transparent.
Give up on an alpha channel and just render the explosions additively. This also doesn't look terrible, but means that explosion bitmaps can only ever brighten the scene, so you lose all the interior smoky details.
So I'm wondering how Freespace handles its explosions.