Command line tool for image conversion

ImageMagick can do format conversions with a tool that comes with it called convert. You can find binaries for it here.

You'll want to run something like this on Windows:

for %%f in (*.jpg) do (
       convert "%%~nf.jpg" -type truecolor "%%~nf.bmp"
)

Or in Bash:

for f in *.jpg; do convert "$f" -type truecolor "${f%.*}.bmp"; done

Convert picks the format automatically from the extension and -type truecolor makes sure you are converting to 24-bit.

Try Convert. It can do a lot.