If you’re uploading images to the web, to ensure they load quickly for your users, you’ll want to compress your images first.
First let’s quickly explain the most common image file formats and what you will use them for.
JPEG or JPG is a common format used for photos. Most phone cameras and consumer cameras will produce JPEG files. These don’t allow for transparent areas, but can be compressed quite significantly. Too much compression will reduce the image quality.
PNG is a better format for graphics. You can save photos as PNG files, but they will usually be huge files compared to a compressed JPEG. PNG files do support transparent areas, which makes them ideal when you need to apply them on top of an image or colored background.
On Windows, a free app called Caesium (https://saerasoft.com/caesium/) is a reasonable choice for compressing JPEG files. While this can also handle PNG files, I recommend you don’t use it for that purpose.
A better choice for PNGs is a free app called Pngyu (https://nukesaq88.github.io/Pngyu/) that is also available for Macs. This is a clever app that can massively cut the size of PNG files, though note that for higher compression levels, image quality may noticeably degrade. If you need photo quality and transparency, a PNG file compressed through this maybe what you need.
For Mac users, you can download ImageOptim (https://imageoptim.com/mac) for free and that will compress both JPEGs and PNG files, though Preview lets you compress your JPEG files too.
Plus a bonus: Animated PNG files
The web is awash with silly animated GIFs, but animated PNGs are another way to create simple animations. These may not work with older computers, but should be fine with most modern machines. They can have better image quality than animated GIFs.
You could just use these as a way to share a joke, but animated PNGs can be great to show how to carry out a series of steps when explaining how to do something.
APNG Assembler (https://sourceforge.net/projects/apngasm/) is the easiest way I know to create animated PNGs. You can download it for free for both Windows and MacOS. The app can also handle compression to keep file sizes down.