ART, AOL Compressed Image File (.art)
The ART Raster image file format
ART files are most commonly associated with America Online (AOL). They are binary bitmap image formats, compressed by AOL services. ART files are saved in a proprietary format used by AOL almost exclusively, hence why most image viewing and editing programs do not support this file format. They can be opened in AOL client software via File - Open and saved in more commonly supported formats using the "Save As..." option.
Technical details of ART files
ART files comprise of single, highly compressed images. AOL developed the image format to speed up downloads of images and web pages containing ART images. During the compression, the image quality may be significantly reduced. Technically, the ART format shows many similarities to the JPEG format.