What a free ChatGPT account gets you
Whether you make a free ChatGPT account, or log in with the same account you've had since 2022, you now have (or will soon have) access to the GPT-4o model. It may be enabled by default, or you may need to switch to it manually. (On mobile or desktop, select the model name at the top of the screen (i.e. ChatGPT 3.5) and make sure GPT-4o is selected.
Once selected, you have access to the same GPT-4o model that paid users do. OpenAI says it's better than previous models at understanding the images you upload, and will have improved responses and insights in turn. In addition, while you should see about the same performance when dealing with English text with GPT-4o versus previous models, OpenAI says it's much better at processing non-English texts. (Check out some anecdotal evidence to see how much faster GPT-4o can be in comparison to GPT-4.)