I needed a quick way to convert many images from jpg to png on Ubuntu and this is what I came up with.
1. Install Imagemagick (for Ubuntu)
sudo apt-get install imagemagick
You can find your system here.
2. Convert jpg to png and keep the original file name
convert *.jpg -set filename:original %t %[filename:original].png
This converts all the jpgs inside the current directory into pngs and keeps the original filename.
You can also set the -quality or convert from png to jpg or what ever you want.
You can also scale the image:
convert *.jpg -affine 0.5,0,0,0.5,0,0 -transform -set filename:original %t %[filename:original].png
This will shrink the image.
You can also resize images and keep the proportions:
convert *.jpg -resize 100x40 -set filename:original %t %[filename:original].png