`Imagemagick' on your linux box.
Example: You have 12345678 pictures [PNG, JPG, whatever] with a resolution of 3072x2304 pixels and you want convert them into a resolution of 1024x768 pixels [these new images will be saved into a new directory].
So, you are [running a terminal] where you have the pictures, you just run this:
$ for files in *.JPG; do convert -thumbnail 1024x768 $files ./lq/$files ; done
In this example I'm converting JPG images [you can choose whatever format if Imagemagick accept it] and saving the new images into the `lq' directory [of course you can choose other directory to save your new images].
Well that's it....
PS: If you found any error in my English just tell me [I need to improve it]