Resizing
Navigate to the directory containing the images (via the terminal) and then enter the following command:
mogrify -resize 20x20 *.png
Which will resize all PNG files to 20×20 pixels.
This can obviously be altered to cater for different sizes and file types.
Conversion
The command to convert a whole folder of images from .bmp to .jpg is:
mogrify -format jpg *.bmp
Imagemagick for Windows
- http://www.imagemagick.org/script/download.php