Resizing and rotating images can be a really time consuming and boring task that just begs to be automated. If you are running Ubuntu, this is easily achieved by using Imagemagick.
To install imagemagick, just do an ordinary apt-get, like this:
sudo apt-get install imagemagick
Now you are ready to start batch processing your images. Here are some examples of useful commands:
To resize an image to a specific size:
convert foo.jpg -resize 640x480 foo2.jpg
To resize using a percentage:
convert foo.jpg -resize 50% foo2.jpg
To resize an image to a specific width, keeping aspect ratio:
convert foo.jpg -resize 640 foo2.jpg
To resize an image to a specific height, keeping aspect ratio:
convert foo.jpg -resize x480 foo2.jpg
To rotate an image:
convert foo.jpg -rotate 90 foo2.jpg
These commands can be chained together like so:
convert foo.jpg -rotate 90 -resize 640 foo2.jpg
To operate on multiple files, file globbing can be used, like this
convert *.jpg -rotate 90 -resize 500 rotated_%03d.jpg
The %03d means three incrementing digits. So in this example, the new file names will be rotated_000.jpg, rotated_001.jpg etc.