16 pixels by 16 pixels is a common resolution for assets matching a low-res pixel art aesthetic. Although the assets may not be specifically 16x16, they are designed to work in a graphical setting where objects are around that size.
Uses two-dimensional "sprites", 2D images created and used on a flat plane, as opposed to the three-dimensional models or environments found in 3D games.
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.
