Save the image, and sync it to your iOS device; one of the easiest ways is to email it to yourself, then tap and hold the image in an email and then tap Save Image. It will be added to your Camera Roll, and you can then set it as the lock screen wallpaper in the Wallpaper settings. Check that the text is readable in this smaller size; especially make sure that, if you put it near the bottom of the wallpaper, it's not behind the "slide to unlock" text that displays when you wake up an iOS device. If not, go back to Preview and move the text so it's in the right location.
Let Siri help you
There's one more way to have your iPhone tell a good samaritan who it belongs to: Siri. If you have Siri set to work on your lock screen (Settings > Touch ID & Passcode, and then scroll down to the Allow Access When Locked section), all someone needs to do is press and hold the home button to activate Siri, and then say "Who does this iPhone/iPad belong to?" Siri replies by displaying information from your contact card, if you have chosen one in Settings > General > Siri > My Info.
There's one small problem with this: Siri displays all the information on the contact card, so anyone who finds your device can know your name, address, phone number(s), date of birth and much more. You may want to set up a second contact card with limited information, but that means that Siri might not be efficient with other requests.
No matter which method you choose, it's a good idea to have contact details on your iOS devices and your laptops. If you lose one of them, a Good Samaritan may come along, and this information could help you get your device back.
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