For the main Portfolio page: the various portfolio entries are arranged in ascending chronological order, meaning newest portfolio entry shows first along the left side of the top row, next newest is to the right of that, then so on showing older portfolio entries as you move from left to right down the main filterable portfolio page. To adjust where a specific portfolio entry shows, you’ll need to edit that individual portfolio entry’s ‘Publish Date’, making it newer or older than the entries around it.

For individual photos within a specific portfolio entry: just drag specific image higher or lower within the ‘Featured Media’ section of the entry. Images will show in order from top to bottom.

Hope this helps.