I just had a stack of 25 .bmp files that I needed to convert to .jpg for a review I’m writing over at maximumcpu.net (Big C Digital Microscopes) so here is an easy way to to that. You can probably do this between just about any two standard image formats. Unfortunately, this does not preserve filenames when bulk converting.
In a terminal:
sudo apt-get install imagemagick
cd ./Desktop/pictures (change this to whatever you need to)
convert ./*.bmp .jpg
there, simple. it pumps out all the files with the filenames just being numbers in order.