When searching for images on Google, be specific. For example:
Searching for "Space" does not produce many good finds.
Searching for "Nebula" or "Crab Nebula" produces great stocks.
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Searching for "Tiger" might not produce what you want.
Searching for "Swimming tiger" might just be bang on.
Hope this helps a little bit.
Also, it helps if you know the specific name i.e. Eagle Nebula, Crab Nebula, Orion Nebula.