I'm feeling pressured
You shouldn't be pressured into doing anything you don't want to do. Sex and sexual activity of any kind is something people do together to feel good. It's not something which can be "owed". You don't have to do anything you don't want to do - even if you are in a relationship or have done sexual things with that person before.
How to stop someone sexting you
With text messages and calls it can be a bit harder to block someone. This means you should always be careful about giving people your phone number. You can block a particular number from contacting you on some mobiles.
WhatsApp and most messenger apps let you un-invite or block users. Check your phone user guide to see if yours can. If it gets too much, getting your phone number changed or buying a new SIM card is another option. A parent or carer could help you with this.
If an adult has been making you feel uncomfortable by asking you to send them images, you can report them on the CEOP website. If an adult does this it is sometimes called online grooming. It is wrong for anyone to be pressuring you in this way. If you are under 18, they are breaking the law.