Image Magick takes care of all my image converting needs. It is a wonderful tool for saving important images from the rot that happens when you move images around that are stored with a compression-minded format.
Ever notice that your .JPG files get all gritty after some time? I sure do not like that. So I prefer to save things in the .PNG format. While the image file-size becomes larger, the quality does not decrease overtime. A trade-off I am willing to pay in this age of external hard-drives and Internet storage.
You can usually find Image Magick in your Linux repositories. If you cannot, or you run Windows, you can pick up binaries on the website at:
There is a laundry list of commands and functions this program suite has. An excellent tool to have around for bash-scripting jobs.
I use this command on my Linux box to preserve my important files. I pack them all together into one directory, and then enter this into the terminal:
mogrify -format png *
It converts all files in the directory to .PNG files. Make sure you only have image files in there.
Additional help can be found here:
Cel Shaded 