If you create your project's folder structure first and then add content in stages then this tool might be of use.
It will let you pick an image file and then copy it to any folder that has no content, or optionally only other folders.
It will make this image the folder thumbnail and if required exclude the image from the album.
Running the tool:
After installing the tool select it from Menu/Tools/External tools/Add default image to empty folders.
You will see this dialogue window
Click on the 'Select image' button and then in the file browser window that opens locate the image to be used.
Check or uncheck the two options, as required, to exclude the default image and if to add it to folders with just subfolders or not.
Click OK to proceed.
When the tool has finished you will see this dialogue window
When you finally add actual content to your folders you can manually delete the file if required and pick a new image to represent the folder thumbnail image.