Your photos are put in the following default order:
- In the timeline: according to the date they were taken, from new to old
- In Albums/Themes/Places: according to the date they were taken, from old to new
You can change the order in Albums under ‘View & Filter’. You can sort by date taken as well as by file name.
When photos are not in the correct order, this is in all probability because the date they were taken was not correctly set by the camera. You can check this by opening the photo in question and then going to image details (i).
If the capture time is not recorded correctly, you can change this in myCloud. There's more information on this in the next question.