Always fill in the alt text field when uploading images. Alt text helps screen readers and also gives search engines context about what the image shows. Require alt text through field configuration so editors cannot skip it.
Use descriptive file names before uploading. A file called red-silk-dress.jpg is much better for SEO than IMG_4271.jpg.
Set up responsive images properly so browsers download appropriately sized versions. This improves page speed, which affects rankings. Consider converting images to WebP or AVIF for smaller file sizes, using the Image Optimize module or a similar solution. Compress images before or during upload to keep file sizes small. Combined with lazy loading, which is available in Drupal through the Lazy module or core support, these optimisations make images fast, accessible, and discoverable in image search.