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 Resizer

To 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 Converter

To 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

Restart Nautilus

Use command nautilus -q in Terminal. Then open Nautilus. This will inform you that the Nautilus extension loaded. If this fails, just relogin.

How To Resize

Once you install nautilus-image-converter, you will notice new items on right-click menu. They are Resize Images... and Rotate Images.... Select images as many as you want > right-click > Resize Images > a window appear > select resolution from menu > resize. You will notice new files created in same folder. Your original images will not deleted.


How To Convert

Once you install nautilus-image-tools, you will notice a new item named Image Tools in right-click menu. To convert many PNG: select many images > Image Tools > convert > a window appear > select destination format e.g. JPEG > OK. You will see new JPEG files created. Your original images will not replaced.