If you want to simply change the image in the sidebar, you must find the coding in the theme (custom html in customize). This is what the coding usually looks like (highlighted in yellow).
If you’re having a hard time finding this code. Use your browsers search (usually CTRL+F) and search for <img src=
To change the image, simply replace the link with a direct link of another image you want to use. To get a direct link, upload the picture to a site like tinypic.com.
To add an image, upload the picture and copy and paste the direct link into this code.