Many graphics formats use or used .art file extension, so it's incredibly difficult to give a particular answer about how to convert art to png. But generally speaking, this is export to common PNG, so any graphics editor might be actually up for this task, provided it does support your particular type of ART format.
Some .art files might be used for vector graphics, in which case art to png conversion will be vector to raster conversion with all the implications and drawbacks.
Updated: August 28, 2023