It is a very recommendable program that allows you to convert size and format of lots of photos in just one click.
Choose a group of files or a folder and set the output settings, Image Tuner will do the rest.
Image Tuner offers different preset sizes (320x240, 640x480, 720x576...) and you can set your custom one as well as the output format: JPG, PNG, BMP, GIF or TIFF.
You can also rename pictures or include watermarks. Don't hesitate, if you want to convert a lot of pictures, Image Tuner can solve your problems... and it's free.