Compress images with image-optimizer (https://github.com/spatie/image-optimizer). Those following dependencies must be installed on the server:
sudo apt-get install jpegoptim
sudo apt-get install optipng
sudo apt-get install pngquant
sudo npm install -g svgo
sudo apt-get install gifsicle
After, you can do that in twig (format in jpg in case the client uploads png images for photos):
<img src="{{ asset.getUrl({mode: 'crop', width: 100, height: 100, format: 'jpg') }}" >