I recently "discovered" caja image converter in Synaptic. You need to install this plugin separately. It works similar to reduceimage in Android and you can resize multiple files easily inside the caja gui. It is working pretty well. You just need to realize that it is slow and seems to work from the bottom upwards. Work with 5 or so files first to understand how it works and how little user feedback you get. Highlight and right click. However, I really like this and I have been searching for something like this forever in Linux.
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Yep, it’s really neat if you are trying to resize icons in an icon theme like from 32 pixels to 48. Of course if you’re trying to convert 250 of them, it’s much better to do it in groups of 50 or so. 
I do it to compress my picture archive which are huge. and yes… 50 at a time is better.