This is because the carousel style is set to randomize.

To change this, find the style you are using on the page, go to onMessage > Website > Site Name  

  1. In Pages & Menus find the page you need to change the photo order on
  2. Click the Edit Wrench 
  3. Click Edit Style
  4. Click the carousel picture, on the left in the Applied Style dropdown menu, it will show you the style used on this page, as shown below.

 

  1. Go back to Pages & Menus
  2. Click the Styles tab located horizontally at the top of the page
  3. Search for the name of the Style, then select Copy to copy the style and make the changes in Steps 4-9
  4. Click the copied Style
  5. Click Edit
  6. Click the Gear Icon to the left of the screen 
  7. Click Edit Settings
  8. Click Randomize Off as shown here
  9. Save & Close

This change will stop the photos from being show in a random order and go in the order you have selected based on their order in the photo category itself.