Wufoo -> Dropbox -> Webapp - How to get user uploaded files from the directory in a gallery montage?

Since Wufoo only have Top level folder integration...
It just occured to me: Can I also grab the content of another folder, NOT located under the Apps folder?

I'm not too familiar with the Wufoo Dropbox integration itself, but my understanding is that it accepts files uploaded to your form and puts them in your Dropbox. It sounds like you then want to get those files from your Dropbox and display them on your site in a mosaic. This would certainly be possible using the Core API, which enables API apps to retrieve files from Dropbox accounts from any folder. Since you're using PHP, we recommend using the official PHP Core SDK. We don't have any samples already coded in PHP that do exactly that, but you can get started with the following resources:

https://www.dropbox.com/developers/core/sdks/php
https://www.dropbox.com/developers/core/start/php
http://dropbox.github.io/dropbox-sdk-php/api-docs/v1.1.x/

How can I change the target folder for the retrieval of users' uploaded photos?
The only thing I did so far was manage to skip the oAuth process by hard-coding the accesstoken. Upon refresh my PHP file, it reuploads the "working-draft.txt" files.

I'll get to reading those docs... but not much "script example" are shown.

What making an API call, e.g., using getFile, you can specify the full path to the desired file. E.g., from the PHP Core SDK tutorial, I might change the getFile line to:

$fileMetadata = $dbxClient->getFile("/Form1/image.jpg", $f);

In this case, I'm downloading image.jpg from the Form1 folder.

I can't seem to be able to "list" the files of a particular folder located on the Root of my dropbox account.
The Path is Valid, but perhaps my App doesn't allow access to this part of my account. How can I fix this?

I'd need to see the error or output to say with more confidence, but it sounds like your app is registered for a more restrictive permission than you need. (E.g., maybe you registered for "app folder" when you need to access other files outside the app folder.) You can find more information on the different app permissions here:

https://www.dropbox.com/developers/reference/devguide#app-permissions

If you need to register for a different permission, you can do so here:

https://www.dropbox.com/developers/apps/create

In fact, my Wufoo uploaded files are outside of the App folder. Do I need a Full DropBox access (All file type checkbox) or the Specific File types one?

Either full or file type access would allow your app access to files outside an app folder, so whichever you prefer.

Great!
I managed to list all the images of my wufoo upload folder into PHP...

Do I have to specifically download every file to my own server or can I directly link to those?

That's one way, or you can use the createTemporaryDirectLink method to get a temporary link to the file contents without downloading the contents to your server itself:

http://dropbox.github.io/dropbox-sdk-php/api-docs/v1.1.x/class-Dropbox.Client.html#_createTemporaryDirectLink