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32 pixels by 32 pixels is a common resolution for assets matching a low-res pixel art aesthetic. Although the assets may not be specifically 32x32, they are designed to work in a graphical setting where objects are around that size.

Pixel Art refers to the charming, simplistic, retro/renaissance style of very-low-resolution graphics, such as from the 1977-1995 Atari, GameBoy, & Super Nintendo (SNES) eras.

PNG, or Portable Network Graphics is a common image file format that is lossless, can contain an transparency channel, and compressed. An ideal format for distributing static assets without any loss in quality.