I recently tried Crunch, an open-source tool for PNG image file optimization, currently on version 2.0.2 at the time of writing.
Lots of web pages now rely on SVG instead of PNG or JPG images. SVG stands for Scalable Vector Graphics, an XML-based vector image format that scales well, unlike bitmap images (bitmaps become pixelated when up-sized). I recently wanted a quick way to convert a SVG to PNG, and discovered modern browsers (read: Chrome and Firefox) can do this with a simple bit of code.
Microsoft PowerPoint 2016 for Mac (aka version 15) has a strange quirk - when I copy and paste images from Preview or most other applications, the image gets copied in Tagged Image File Format (TIFF). The image is practically uncompressed (or possibly minimally compressed), resulting in very, very huge PPTX files! Plus: Removing unwanted font references.