NConvert is a command line tool for image batch processing.
The program can crop, resize and rotate images; tweak brightness, contrast and colour; apply sharpen or blur filters, add watermarks, convert images formats and more (80+ commands in total).
NConvert can work its magic on just about any file, too. The program reads more than 500 image formats, and can export to more than 70.
You will have to spend a little time learning how NConvert works, of course, but it's really not too difficult. Here's an example.
nconvert -out gif pic1.jpg
You can probably guess that this takes the file pic1.jpg and saves it in the GIF format.
Let's get more advanced.
nconvert -out gif -dither -colors 256 *.jpeg
Now we're converting a set of JPEG files to GIF, and defining how the colour palette will be created.
And now try this.
nconvert -out png -resize 200% 200% *.jpeg
There's no need to spell this out: the English-like syntax makes it all very obvious.
As you can see, there's no need to be put off by NConvert's command line origins. There is some work to be done in understanding its features, but you'll figure it out quickly enough, and there's more than enough batch processing power here to justify your efforts.