I just spent a few minutes looking for a tool to convert a small number of PNG files to JPG on Mac OS. It was just a few dozen files but so in the end fell back to the good old command line because I don’t like graphical interfaces for these tasks.
For future reference:
for file in *.png; do jpg="$(basename \"$file\" .png).jpg"; convert "$file" "$jpg"; done
Yes, it’s that simple once you have installed ImageMagick.