You probably noticed that there's a 3MB limit for image uploads which in itself is quite large in terms of loading times. Ideally, these images can be reduced in size to much less so that they load quicker. Don't confuse resolution with file size or weight. Even a 1920 x 1080 image cxan be reduced to a reasonable size like 500KB for instance.
I use TinyPNG and TinyJPG which is an online image compression service. Simply upload the file, wait for it to compress and download the result. A 500KB image will load much faster than a 1MB image.
For more in-depth image manipulation I use Faststone Image Viewer which is also free and has some very useful tools, such as batch conversions for resolutions and sizes.
Remember that generally speaking .PNG images are usually larger in file size than .JPG, so wherever possible it's good to use .JPG. For non photograhic images such as screenshots of web pages or technical charts, drawings etc or stuff that contains a lot lines, PNG images give a clearer result.
For screen capture I use Snagit 12 because you can then edit the screenshot with arrows, text and all sorts of other methods. Snagit will also record your screen as an .MP4.
Windows also has the free Snipping Tool which on Windows 10 has now been updated to Snip and Sketch with many improvements.
If your image is very large, try to reduce the resolution dimensions or if you can't do that, post it as a thumbnail, otherwise it takes ages to load and takes up an enormous amount of screen space.