The quick and dirty way to change it would be to replace the icon file (blog-empty-placeholder.png) in the directory <Confluence_Install>/confluence/images/icons.
Here is a discussion of how to use CSS to customize icons:
Two vulnerabilities have been published for Confluence Server and Data Center recently: March 20, 2019 CVE-2019-3395 / CVE-2019-3396 April 17, 2019 CVE-2019-3398 The goal of this article is...
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