Imagemagick is hiding in your pclos machine. It's an entire suite of applications that can process photos, view photos, build thumbnail pages and obtain photo information. It only works in the command line, so the likelyhood is that you don't even know it's there. You can use from the command terminal or you can use it from scripts. It even has it's own scripting language, but it gets along very well with bash.
Would you like to make the background color of a .gif transparent?
$ convert -transparent '#fff' input.gif output-trans.gif
where a copy of a file named output-trans.gif with a transparent white background "#fff' is made from a file input.gif which has a plain white background.
Want to resize all the images in a folder?
# mogrify -geometry '800x600>' -quality 60 *.*
will re-size all the images in the working folder to have a size not exceeding 800px wide or 600px high while retaining the aspect ratio and file name.
You can find out all about it here.http://www.imagemagick.org/
If you're not into writing scripts there are many available on the web. IM is often included as part of other applicaitons and has even been ported to Windows.