Hi Sophie,
When you collect data in a survey with Survey123 for ArcGIS, behind the scenes, you are storing your data in a hosted feature layer. This will include the photos in your survey, they are saved as attachments.
To view the photos, simply add your hosted feature layer (this is the feature layer in your survey folder) from your My Content page to a new web map. Then just click on a point feature in the map viewer and you will be able to see the photo in its pop-up (there will be a link to open the photo).
Hope this helps,