Emacs has built in support for XBM and PBM/PGM/PPM images. This is sufficient to see the monochrome splash screen and tool-bar icons. Since v22.2, the official precompiled binaries for Windows have bundled libXpm, which is required to display the color versions of those images.
Emacs is compiled to recognize JPEG, PNG, GIF, TIFF, and RSVG images also, but displaying these image types require external DLLs which are not bundled with Emacs. See Other useful ports.