The most common issue is that an image has too high a resolution for the area that displays it. This forces the browser to resize the image to fit the screen or area. If the size of the file is unnecessarily large, then it will take longer to be transferred over an internet connection. The browser will then throw away most of the information. You should resize the image ahead of time before adding it to the site.
There are many image manipulation tools available to resize pictures. If you run Windows, then Paint.NET (https://www.getpaint.net/) is an excellent free piece of software. This is much better than the Paint program that comes with Windows (although, this will work if you have no other option).
For other platforms, ...
