Here is how to convert a set of PNG images into an animated GIF:
# convert all PNG files into GIF
for file in *.png; do
convert $file $(basename $file .png).gif
done
# Combine them into an animated GIF
convert -delay 30 *.gif animation.gif
The "convert" command is from the Imagemagick package. It's a Swiss army knife for images.
I can recommend animated GIFs for research purposes as opposed to presentations. With gifview (included in the gifsicle packageEstanislao recommends), you can use an animated GIF as a slide show: you can "play" it normally or "step" it (frame by frame) forward or backward. For that reason, it is sometimes useful to generate an "animation in the x direction", showing a series of meridional sections as a slide show, etc.
The downside is that compression in the time direction isn't great because it's basically a slide show.