Hello van_peter,
The first thing we would suggest would be to power cycle the iPhone by pressing and holding either volume button and the side button until the power-off slider appears. Drag the slider and wait 30 seconds for the device to turn off. While simple, this can refresh the iOS, apps and connections. More information is available here: Restart your iPhone - Apple Support
Once restarted, check to see if they appear.
If not, it is possible they were part of a Message thread that has been deleted. They may also reference photos from your Photo album that are no longer accessible. If you are not able to tap on them to see them, they may have already been removed.
Since you are not able to see what they are, we would suggest that you create a backup of all the data on the iPhone using one of these methods: How to back up your iPhone, iPad, and iPod touch - Apple Support
Once backed up, you can delete the images. If you find you deleted something you need, you can restore the backup to retrieve it.
Have a good day.