So, imagining that they have been converted does not work
For batch conversions you can try Graphic Workshop. Suggest you Google gwsw95.exe to find the original Win95 shareware release. (The 1864Kb version self-extracts entirely into its own directory. The installation program may hang after it has completed its work but never mind. Until registered this version exits with a chime and a written reminder to register and get good karma.)
To convert from ART to JPG use the File menu to change to the directory of the source files. Click on the *.art files that you want converted, click on the icon for conversion - it is the leftmost one on the toolbar under the File menu, then click on the JPG button as the target. It will leave the originals intact and create JPG copies in the same directory as the originals. I have never handled the *.art format so I cannot attest to the quality of the conversion from this format.
If this is a one-time need then just delete the entire directory when done (if you downloaded the version mentioned above). For longer term use you can obtain the current version from
http://www.mindworkshop.com/alchemy/gwspro.html