Choosing stock images
Unsplash, Pexels, StockSnap, and Negative Space focus mainly on photography. Pixabay also includes illustrations, video, and audio. Shopify's collection is worth browsing for product and commerce imagery. Search by subject, then compare the composition and crop options before downloading.
Read the current license on the source site, especially for images of people, brands, or products. After choosing an image, resize and compress it for the page, write useful alt text when the image conveys information, and avoid using the same generic photo across many pages.