You will most likely notice this as a missing image and a strange looking URL somewhere in CMS, such as this one:
This suggests that your website was unable to locate appropriate image in its Theme, located in App_Theme subdirectory, and tries to unsuccessfully load it from the default image archive. The most likely reason is that the codename of the CSS Style Sheet used by your website has changed, and no longer matches the App_Theme subdirectory name, or the CSS Style Sheet has been completely unassigned from your website.
To fix this, make sure that a CSS Style Sheet is assigned to your site in:
CMS Site Manager -> Sites -> Site CSS stylesheet
And that its codename exactly matches the App_Theme subdirectory:
CMS Site Manager -> Sites -> Site CSS stylesheet -> Edit -> Stylesheet code name
If this checks out, see if the missing image physically exists in the App_Theme subdirectory and is accessible using web browser. If the image exists but is inaccessible, try reapplying document permissions to the entire directory.