How do I recover pictures after mistakenly selecting 'I lost my iPhone'?

Turning Mark as Lost an iPhone will not delete any data.

Turn off Mark As Lost on a device

When you find your lost device, turn off Mark As Lost.

1. In the Find My app  on your Mac, click Devices.
2. In the Devices list, select the device you marked as lost, then click the Info button  on the map.
3. Under Mark As Lost, click Pending.
4. Click Turn Off Mark As Lost.
5. Click Turn Off to confirm.

You can also turn off Mark As Lost from your iPhone, iPad, iPod touch or Apple Watch by entering the device’s passcode (the one you use to unlock your device).

Note: You can’t turn off Mark As Lost remotely from another Mac. You can only turn it off directly on the Mac that you marked as lost — by entering the numeric passcode you set up when you marked your Mac as lost.

If you forget your device passcode, you can recover it using Find My iPhone on iCloud.com. See Use Lost Mode in Find My iPhone on iCloud.com in the iCloud User Guide.

Regarding your data…

1. If you have an iCloud or iTunes latest backup, then there is nothing to worry about. Just restore from it, else...
2. If You have synced your data to iCloud, then what you will get back is what you have synced to iCloud and uploaded to iCloud Drive;
   1. The information that is synced and stored in iCloud, like Contacts, Calendars, Bookmarks, Mail*, Notes, Voice Memos, Messages*, shared photos, iCloud Photos*, Health data*, call history*, and files* you store in iCloud Drive.
   2. *-If you had turned on Messages in iCloud, iCloud Photos, Mail, Health data, call history under iCloud, and files you store in iCloud Drive your content is automatically stored in iCloud and is available on all devices that use the same Apple ID
3. You have imported your Photos & Videos to Mac/PC then they are safely residing there on Mac/PC
4. You have uploaded some of it to Cloud Drives using the Files app (iCloud Drive, Google Drive, OneDrive, DropBox, Flickr, box, AmazonDrive, Adobe Creative Cloud etc). The data remains there.
5. If you have not backed up or do not have the latest backup, nor imported nor you have synced or uploaded your data to cloud drives prior to the situation that you are in, then I am sorry, all your data is lost.

This may help --> Do's and Don'ts for lost iPhones - Switch… - Apple Community