If you’re posting someone else’s work, credit them. Don’t deliberately remove the name of the artist. That’s just wrong. If your reasoning is that it’s own personal scrapbook — then keep it personal, not public. If it’s public, the least you can do is credit the artist and link back to their site, or at least their Wikipedia entry.
When I see something I like not credited — I try to track down the source, if for no other reason than to find more art by the same person. Consider this a small payback to the artist whose art you liked enough to post on your Tumblr. Use Tineye to track down original visual art — it’s easy to use, and takes no more than 10 seconds to do a search.
And that was your PSA for today. In next week’s episode: public shaming, followed by stoning.