Online tools for instant image compression

https://www.websiteplanet.com/webtools/imagecompressor/

Image Compressor - Compress your PNG/JPG files by up to 80% and retain full transparency

 

https://imageresize.org/compress-images

Advanced PNG and JPEG compressor for reducing file size without quality loss, additional tools available at the site: Image resize, Bulk resize and other PDF and image conversion tools.

 

https://tinypng.com/

Smart PNG and JPEG compression

 

http://compressjpeg.com/

Allows multiple file conversions (up to 20 files per job), compresses JPG, PNG and PDF's

 

http://optimizilla.com/

Yet another solution, similar to compressjpeg.com