I have added
convert %f %f.png to Thunar's custom actions to convert a selected image to png.
What would be the command that can be added in the same way that would allow selecting multiple files or an entire folder for the same purpose?
I use Thunar to convert multiple svg files to various PNG files - mainly used as icon files for the Xubuntu desktop environment.
For this I created a script, based on another script I found somewhere. As this script converts a set of selected images files into other image files, which might help you as well. Here is what I did to get this working in Thunar;
This will convert the image file (in my case, the svg file) into two PNG files, one with size 96x96 and one size 64x64, into two separate directories.
With a bit of tweaking, you can use this script to suit your own wishes (e.g. not changing the size, creating various directories for the files) etc.Just change the script I pasted above, and play around with it a bit.