If you want a powerful batch image converter with lots of output file formats support then Converseen is an excellent choice (including RAW image support). Or the “Sir” image resizer is also pretty good one too.
But if you’re looking for a tool that is even more simpler (but useful!) then you should try this other tool called “Rarig”. It doesn’t have some of the options those above mentioned tools have, but I think it’ll be able to stand up to most of our needs :).
Few main features …
*. Supports both Ubuntu and MS Windows.
*. Batch image resizing with renaming.
*. Easily change the name Prefix.
*. Output formats supported: JPG, BMP, PNG, GIF and JPEG.
*. Change the quality of the output image.
*. Optimize the image (not sure what that does actually. Perhaps a meta-data optimization).
*. Change the image height (to keep the correct aspect ratio Rarig will automatically calculate the width).
*. Create a compressed (archives) file.
*. Choose an output folder.
If interested, you can install Rarig in Ubuntu 12.04 Precise Pangolin, 11.10 Oneiric Ocelot, 11.04 Natty Narwhal, 10.10 and 10.04 by first getting the pre-built “.deb” package from this official Rarig download page (including packages for MS Windows users too).
Then as usual, once the download completes double click on the file and Ubuntu Software Center should install it for you. It it gave me a warning saying that the package is of bad-quality but it’s working without any issues in my 11.10 though. Good luck.