This article lists image resources for your posts and pages.
Image sources
These sites provide royalty-free images:
You can find more image sources listed on WP Beginner.
Image sizes
We recommend selecting a high quality JPEG images under 5 MB large, with your preferred aspect ratio If you plan to share your post on social media, select an image that fits with that network's recommended guidelines.
Setting images in posts and pages
We recommend using the Featured Image menu to add images to a post or page.