In Dolphin, by using an extension, I can do right-click to convert bulk PNG images into JPEG or any format. This is very effective (I use it very intensely) for my graphic design works. But how about Nautilus? Sure, you can do the same thing. You will need to install programs nautilus-image-converter (resizer) and nautilus-image-tools (converter). nautilus-image-converter created by GNOME team and nautilus-image-tools created by Atareao Team. Thank you!
Install The ResizerTo do this, install nautilus-image-converter from Ubuntu repository. I'm using Ubuntu 15.04 and the package is already there. Install it using Ubuntu Software Center or by Terminal:
sudo apt-get install nautilus-image-converter
Install The ConverterTo do this, install nautilus-image-tools from unofficial repository. Atareao has created that unofficial in Launchpad PPA. Use these commands:
sudo add-apt-repository ppa:atareao/nautilus-extensions sudo apt-get update sudo apt-get install nautilus-image-tools