There are many tools you can use, but personally I use the free/open-source tool
GIMP. You just open your image file, use the "Scale Image" menu item to scale it down (if necessary), and then export it as a PNG or JPG file.
Just doing that should reduce the file size a huge amount (with the exact amount, and the exact impact on your image's detail, depending on how far you scale it down).
P.S. Also there's a way to batch process several images at once using GIMP, but I've never done that:
https://ulyssesonline.com/2008/09/22...ges-with-gimp/. And if for whatever reason GIMP doesn't work for you, there are also websites that do this sort of thing (but usually with some kind of cost: either they charge you or they add a watermark to your images).