Look into image compression Most image handling software that can save files allows you to do it to some extent (by setting a quality level lower than 100% in the options when saving the file), regardless of OS, but there are also several online tools for it as well as specialized software.

Btw, we're not talking about compressed archives (like zip files) here, rather causing a 'loss of quality' in the image to drastically reduce its file-size. In reality most of the time that loss of quality won't be noticeable by the naked eye, unless the original image was really high quality and you actually zoom into it or you want to compress it an unreasonable amount aiming for a really small file, so it's safe to do. 

If done properly.. you can roughly reduce the file-size of a ~3MB photo down to 3-400KB, while keeping the original picture size (if you want to) and there still wouldn't be a loss of quality so significant that it would be bothering in any way. 

Here's an example: Made a picture of the back of my violin with my phone.. it resulted in a rougly 3.5MB file: https://drive.google.com/open?.....zXeHmAgjE9

I compressed it so that it loses roughly half of its quality, but kept the original scale: https://drive.google.com/open?.....Ft02r2HTAi it's now only 500KB, but it's still not bad to look at  

Now.. because the image is actually too large for the screen anyway.. I can also scale it down to be 30% of the original.. while doing the image compression at the same time resulting in a still good-to-look-at file that I can simply upload here as it's merely 85KB (Edit: it WAS 85KB, but apparently the site adds some information to it when uploaded making it 92, if I re-download it it's still larger than the one I uploaded so it's not a counting issue haha) 

Hope this helps