For around 20 years or so now the small program Irfanview has be around. As a windows application it’s purpose is to play with graphics, and it does so with the intent and purpose of making them better to use on the web.
Simple tasks such as resizing, cropping, changing the colour contrast, gamma, or saturation, are easily achieved. Changing the graphic format to some better for the purpose on the web (especially jpg) is very simple.
It is a program that I use most days, and If you are getting pictures ready for the web this is the fastest and easiest way I know how.
There are a whole range of other things that can be done in irfanview, however starting with all the basics it is my basic go to program fro all these tasks.