I believe Dropbox makes tablet apps, once installed these allow you to save images locally to the device such as Camera Roll or Gallery. Once saved locally, Photoshop Touch would be able to import them via local photos. There is no direct method inside Photoshop Touch unfortunately.
You can also use a file manager that has access to cloud storage. Then just move the files to your device. I wish it had dropbox acessibility but I doubt that will ever happen unless someone else writes an a plugin. It would take people away from the Adobe Cloud. There's also drop box downloader which downloads any files from dropbox. It has no automactic feature so you have to manually click the file to download. It's basically the same thing as using a file manager.
I use Solid explorer. It has access to all my cloud storage accounts, has dual windows, and drag and drop. There are a lot of other apps that have cloud storage access.