Picture Resize Free Online Image Converter supposedly supports hundreds of image files, including ART format, so it is your best bet for conversion to JPG format.
Convert Various graphic formats to JPEG bitmap image format.
We found 2 software records in the database related to .art to .jpeg file format conversion.
Microsoft Windows software - convert art to jpeg on Windows
A graphics editor, illustration tool and web design from Xara
Convert from art file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned art to jpeg. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
On-line services software - convert art to jpeg on web
An online graphics converter and resizer