Adding pictures to you Moodle site is great, but they need to be the right size. However the right size has two meanings:

1) Looking the right size on the page in relation to the other images and page elements.

2) The image file itself not being so big that it slows down loading of the page.

If you used the internet in the early 1990s you will remember having to wait while images on webpages loaded, and if you went to a site with big images the wait could be very long. This has been largely solved with faster internet speeds, but the the problem is resurfacing because images are getting better quality. A picture taken on a phone can now be 4Mb in size which will take about 2 seconds to load on standard broadband. Not too much of a problem but if you have 10 such pictures on a page you are into a noticable wait for the page to load. 

However, you might not notice this problem yourself because once your computer has downloaded a page from the internet once, it usually keeps a copy of big images so that it will download the next time much quicker. 

So you need to set the image size at an appropriate size for what you want the image for. 

Here is a video that explains it and talks about resizing pictures. The example here is on a Mac, so see the video below on how to do this on Windows

Finally you might have a large collection of pictures you want to resize. If you search for "image resizer for windows 10" you will find a free tool that you can download that will allow you to do multiple pictures at once. Or you can use this free site https://imageresizer.com/bulk-resize