Just a note the website linked above with photo guidelines is NOT the official ETA website. You can use it to apply but you will pay more.
Are you trying to take the photo as you go through the application process? If so you might first want to take a few photos on your phone, then start the application and upload the photo when you get to that point. Delete any that have been rejected already so you don't accidently try those. As noted, very good lighting, a neutral background, straight on eyes to camera, and looks similar to your passport photo should improve your chances.
If none of that works, save the photos on your computer and use the web form to apply, it's the green button halfway down this page: https://www.gov.uk/eta/apply - keep clicking "I can't use the app" choice until it lets you apply online.