ImageMagick

Convert all images in a directory from one format to another

This is easy. Use Mogrify, not Convert. The following example converts all PNG files to JPEG:

mogrify -format jpg -quality 85 *.png

I always forget this. Maybe it's because the ImageMagick docs say:

Mogrify overwrites the original image file, whereas, 
convert writes to a different image file.

Which is true unless you want to convert a group of images. In that case, you use Mogrify instead of Convert. Why? I would call this a bug. If you try this with Convert it will not work (and will, in fact, destroy the last image in your directory).