From the Model Mayhem FAQ:

How do I organize or rearrange my photos in my portfolio?
Did you know?
If you are using View All for your portfolio, and you have any albums, the view order of your images will be determined by the name of your albums and the order of the images within those albums - so images in an "Azure" album will appear before images in a "Blue" album when utilizing View All.

NEW! Quick Sort

Want to quickly sort your images by upload date or caption? Now you can, and it's easier than ever.
  1. In Photos, start on "Edit View" and select your Sort preference.
  2. Then, click "Order View" and your images should now be ordered in the same order outlined in "Edit View."
  3. Move one or two of your images around to ensure your changes are saved.
  4. And that's it! Check out your new image order in your portfolio!
  5. Check out the video below for a quick tutorial on how this should work.

Moving Individual Images: Drag and Drop Your Images!
  1. Select "Order View" - this will should show you the current layout of your portfolio. (Be sure you choose how many photos you want per page!) Here's a screenshot of where that button is:

  2. Using a mouse, click and "drag" the images where you want.
  3. Simply drop the image where you want it to appear in your portfolio!
Pro Tip! If you want to move an image more than just a few rows, click on it and hold the mouse button down. Then, while holding your mouse, click the "Page Up" key to move up a few rows at a time or the "Home" key to move all the way to the top. Then just make sure it's settled into the right position before you "drop" it.

The system should automatically save your changes, which should instantly reflect in your portfolio.

Here's a screenshot of Drag and Drop in action:

It has been brought to IB's attention that some users with large numbers of photos have a difficult time uploading new images, as the images appear to "scramble" out of order in your portfolio as soon as you upload a new image. (The new image goes on top, the other images appear to be auto-sorted by upload time.)

Thanks to the work of some of our users, a workaround has been found!
  1. After uploading your images, manually swap the positions of any 2 images
  2. After a moment, simply swap the images back
  3. Your previous photo order should be restored and your portfolio should appear the way you sorted it