Mass renaming of batch conversion is frequently used, either for converting your holiday photos or dealing with a massive number of files. Here is how:
Imagine that you have several files to rename by including another extension
(mv fileX.txt fileX.txt.bak).
for i in `ls *.txt`; do mv $i $i.bak; done
However, if you need to batch convert your photos from jpg to png, try this:
for i in `ls *.jpg`; do convert $i `echo $i | sed "s/.jpg/.png/g"`; done