First you’ll need to know whether the photos you wish to delete are stored internally (inside Context Camera) or if they exist in an export location like the photo library on your device. The top-level screen on the photos tab shows all photo locations under internal/export headings.
To delete a photo stored in an internal album, open the album, tap the “Select” button, tap photos to select them, then tap the delete button. Photos cannot be recovered after they are deleted.
Photos stored in an export location (like the photo library or iCloud) cannot be deleted from inside Context Camera. You will need to use a different app to delete them, like the Photos app to delete photo library photos or the Files app to delete iCloud photos.