It looks like Motion "respects" iPhoto's settings for sorting. If you go into iPhoto and set up your events to be sorted in a specific (ascending/descending) date order and restart Motion, Motion will rearrange the list/thumbnails to however they appeared in iPhoto.
Another way to go is to simply pick an image in the same event (/Theme), right click and Reveal in Finder, click the list icon in the Finder window, select the Date Modified column to sort your images by date (the column header will turn "aqua" depending on your system preferences and a small triangle will appear on the right side -- clicking on the selected column header will toggle between ascending and descending order) , find the one you're looking for and click/drag it onto the canvas in Motion (you can make multiple selections in the Finder window and drag everything at once into Motion).
Thanks for your response. The iPhoto sequence is only respected when the library item display is set to "thumbnail" when Motion starts. When set to "list display" (either at startup or during the session) the items are sorted in alphabetical order and cannot be resorted in date sequence anymore.
Of course I can bring photo's from iPhoto to Motion using drag & drop, but that requires iPhoto to be open and I want to avoid that, given the amount of memory both iPhoto and Motion require. And, yes, dragging & dropping via Finder can be done as well, but my point is, this is a bug or omission in Motion, in my view. Motion should not resort when switching between thumbnail and list view. Nor does it seem to be possible to add columns to the library-item view, such as date.
Ok... well... it's actually "in there" but not like you think, and not really very intuitive.
Go to the "Project Pane" (layers list, etc...) and at the top of the column, select the Media tab.
Go to your photos library and select the "theme" (subgroup) your images are in. Thumbnails or list view, doesn't matter — select the pane the images are in and type Command - A (select all).
Drag ALL the photos into the Media pane (they are NOT added to the layers list.)
Right click on any of the column headers (Preview or Name is good) and from the dropdown, select Sort By > File Created (or Modified).
Select the image you need and drag it onto the canvas (it will be added to the layers list.)
I will grant you it's a little strange... Also, BE CAREFUL!!!! Whatever you do, you DO NOT want Motion to Save Unused Media with the project!!!! Make sure that option is unchecked. Or, command or shift select all unused images in the Media tab and delete them before saving.