The problem is that your source PNGs have an alpha channel which is not supported by GIFs. So you have to remove transparency from your source images first. Since you’re dealing with multiple source images, you can’t use the -flatten method. With newer ImageMagick versions the following should work:
convert -background white -alpha remove -layers OptimizePlus -delay 25x100 ps1-*.png -loop 0 ps1.gif
If your version of ImageMagick is older than 6.7.5, you can try:
convert -bordercolor white -border 0 -layers OptimizePlus -delay 25x100 ps1-*.png -loop 0 ps1.gif
I got the following result with the latter command:
