.png-gif? Because they provide you lossless compression of your images, prioritizing color saving than file size compression, resulting in bigger files. jpg-jpeg formats will greatly compress your file size but this will greatly disrupt image quality at low scales and remember, pixel-art its all about working low scale.

Back to .gif-png, both providing big file sizes, while .gif files will let you animate, .png supports just as more colors as .jpg-jpeg files, simple as that.

Now, there's the .bmp option, which will save all the colors, just like .png, but compressing nothing, resulting in supposedly gigantic file sizes.

@ascimat, I'm eager to see you one day posting on #pixelart. Remember to practice, practice, practice.