If you changed the blog-default image and did not retain the filename or opted to provide a new filename, you will need to update the image-default format.
Important: Maintaining proper code syntax is crucial when editing filenames and alternative text in velocity format files. Please make sure to maintain proper code syntax.
Instructions
In the Site Content tree on the left, click on the _cms folder, then click on the formats folder.
Select the image-default format.
Click Edit.
Update the image filename and alternative text. The first line of code contains the image path and the second line of code contains the image alternative text. If you have uploaded your new default image in the same folder, all you need to update is the file name in the first line of code.
Click Preview Draft. After previewing the draft, click Submit to save the image to the CMS.