Oh dear Polidor; I'll try to explain your points to help others but there's a lot to learn here.
1. New phone uses smaller sized SIM card, so old SIM card could not be put in it. The SIM card holds very little info (certainly not your photos, and probably nothing other than your phone number).
2. A "SIM free" phone still has a SIM in it, but can accept ANY card (of the right size), from any service provider - so you can change networks at will, if you want to.
3. Your Whatsapp photos are stored by Whatsapp ("in the cloud"), and can be restored to your phone if you want.
4.Your "ordinary pictures" - I assume these are photos you have taken with old phone - these are saved in the phone memory. They MAY be on an SD card in the old phone, which can be recovered, or maybe not (depends on old phone). You need to restore these from your backup - what do you mean "I don't have a backup"
5. Your PAYG credit could have been transferred, but may be lost now.
ALL photos stored on a phone WILL finally be lost - either the phone, or memory card will ultimately break, or be lost. You must have a backup of photos and other stuff - and you must start doing this today.
Google, Apple, Samsung and all the rest offer no charge options to back up your photos. You don't need a PC/Mac to do this, but you do need good WiFi connection.
