Images usually make up most of the weight of a web page. A single unoptimised photo straight from a phone can be larger than the entire rest of the site. Visitors on mobile data will leave if a page takes more than a few seconds to appear, and search engines also prefer fast pages.
Do not upload a 4000 pixel wide photo to display it in an 800 pixel wide column. The browser still downloads all 4000 pixels and then shrinks it. Look at the largest size the image will be shown, and resize to about twice that for sharp screens at most.
More detail in our JPG vs PNG guide.
A good target for most photos is 100 to 200 KB, and for small thumbnails under 50 KB. Use the EasyLife File Converter to resize first, then compress to a size target. Quality around 70 to 80 percent is usually invisible to viewers.
Use short, descriptive, lowercase names with hyphens, for example red-ceramic-teapot.jpg, not IMG_20240615_0042.jpg. It helps organisation and gives search engines a clue about the picture.
Alt text is a short written description of the image, such as "Hand painted red teapot on a wooden table". It makes your site usable for people with screen readers and is shown if the image fails to load.