At the top right of the News Webpart, click on See All. This will take you to all of your News posts.
Click on Manage Posts, and you will see a list of all of your posts.
Click on the name of the one you want to edit, and it will open as shown in the example below;
Click Edit, and you have the option to Add Thumbnail if you don't have one set, or to Change if you do already have one.
Click Update News to complete the process.
Hope this helps.
You should be able to go to the Site Pages library and from there, click on the News Post you entered. When you click the link from Site Pages, you will get to the link information where you can add or replace the thumbnail or text.
If that doesn't work, try copying the link and replacing everything after the .aspx? with stay=true.
Accepted Solutions
At the top right of the News Webpart, click on See All. This will take you to all of your News posts.
Click on Manage Posts, and you will see a list of all of your posts.
Click on the name of the one you want to edit, and it will open as shown in the example below;
Click Edit, and you have the option to Add Thumbnail if you don't have one set, or to Change if you do already have one.
Click Update News to complete the process.
Hope this helps.