Oooh, I love me a Metal Slug thread! Lots of great behind-the-scenes stuff here I hadn't seen before.As for the OP's question, I'm sure they had a graphic editing program that allowed them to flip between frames or layers. Also, graphics tablets have been around in some form or another for decades, and Wacom (a Japanese company that specializes in graphics tablets) was founded in '83. I don't know if Nazca Corp. was using their tablets, but I'm sure their artists had something similar.After that it's just a matter of actually doing the animating. Here's a video of someone animating an explosion somewhat reminiscent of the ones seen in Metal Slug. Note that the process is very similar to the tree example Lord Error posted in the first page.And just for kicks, here's the elephant slug. A sprite that appears in only one game, for roughly 3 minutes and is never seen again, but which has the most lovingly crafted detailed animation of stretching and wrinkling elephant skin ever seen in a videogame.EDIT: Holy shit, I hadn't heard of In the Hunt before this, but after seeing it now, it's clear how Metal Slug 3's under water stages were meant to be an obvious callback to fans of the team's earlier work.