Can I learn how to photoshop^🚫TM images without PhotoShop^TM and also on my iPhone^TM?

You can try Adobe Express in the app store or use an online editor like http://pixlr.com/ both support layers and will do what you describe on an iphone. Pixlr has some new 'ai' stuff i haven't used that might make it even easier depending on your familiarity with these kind of tools. Good luck!