For batch image conversion, file renaming, etc, I usually use these kind of one-liners:
$ for i in `ls *.bmp` ; do echo convert $i ${i%.*}.png ;done
convert screendump_01.bmp screendump_01.png
convert screendump_02.bmp screendump_02.png
convert screendump_03.bmp screendump_03.png
When you’re satisfied with the result, have it executed by the shell:
$ for i in `ls *.bmp` ; do echo convert $i ${i%.*}.png ;done | sh