Why do some images that look great as thumbnails become blurry or pixelated when enlarged?
Images are created with a FIXED number of pixels. But the crispness of an image display is dictated by the pixel density (pixels per inch).
When images are enlarged, the pixel count remains constant, but the pixel density (quality) decreases.
If you have access to photoshop, or other professional image software - it is possible to increase the resolution (ppi) of images with artificial intelligence (the software uses the surrounding pixels to guess what the new pixels should contain). But the rest of us will need to look for an image with higher resolution or take and upload our own digital picture.
Should I be worried about Copyright Infringement?
Yes. It's super easy to do an image search on google and find every site using an image within seconds - and copyright holders regularly do just that. Of course, there's also the law-abiding, do the right thing argument. . . Fortunately, there are tools available to help you find and identify images that you're welcome to use: https://support.google.com/websearch/answer/29508?hl=en and https://en.wikipedia.org/wiki/Wikimedia_Commons