Setting aside the moral issue on respecting the rights and interests of creative artists given the zillions of blogs out there, including popular ones operated for profit and still using whatever images they can find without even crediting the sources you might not think it's big deal.
I didn't think so myself until I found myself working at site where the editors administrators had received letter from lawyer threatening to sue, demanding payment, and putting the future of the site itself in peril just for the costs of defending themselves legally.
Colleagues and I have had great experiences with ImageInject, plugin by Thomas Hoefter and WPscoop that makes searching, using, and crediting images from Flickr and Pixabay and fun.
You're probably safe with old advertising and fine art, with publicity images generally, and often with screen captures used for critical purposes i.e., to criticize for its coverage, but not to steal their images or graphics but at that point you're starting to get at gray areas and danger zones, as there are potential exceptions to every rule.
On the the other hand, there are lot of photographers and artists out there.
Leaving image errors in place will produce slow loads on affected pages.
As noted elsewhere for instance, under Usage Notes and in the description of this plug-in you may need to clear site and other image caches, including Content Delivery Network caches.
In some instances for instance with JetPack's Photon Module, which does not include option de-activating it for couple of weeks seemed to solve the problem on re-activation, but this experience should not be taken as You may need to try some other CDN, or go without one for while, if you want to use WP-RUBI..
OK, but what if I want to have one kind of replacement image for one type of post, and one for another, or select from gallery of images and what if I want to divide up different time segments intead of just using one timespan, and and..
Let me know, and I'll think about adding the capacities and options to version of the plug-in or I guess we can discuss job if it's urgent ( Though that thing about selecting from gallery you could already get if you linked to a rotater.
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