While Microsoft has done an excellent job of finding stock images that work beautifully within the image size guidelines for SharePoint Online (https://symp.info/ImageSizes), sometimes you need something a bit different. These are my top FREE image sources:
Using an image without a background means your SharePoint theme can really shine as you change section backgrounds and don’t get that pesky white box around your image.
–StickPNG: must create a login and there is a limit of 3 per day with the free account
Last but not least, using the icons already available in SharePoint can be very impactful. It keeps your iconography consistent which can build trust in your content as more aesthetically pleasing designs are viewed as more usable (The Aesthetic-Usability Effect (nngroup.com)). Chris Kent made a wonderful solution for the community called Flicon (Flicon – Fluent UI Icon Search) that improves the keyword search to find these icons and then enables you to export them. Add these icons in as an image to your site for a beautifully branded experience with clear visual cues: