Importing multiple images from Dropbox with Feeds
I hope you can read the images's urls inside the Excel table – or actually let your D7 install read them? Once you have that and can actually also access the files on dropbox, it should be possible to load them into your D7 install. 

I had a similar, though probably more trivial problem. I had the images' urls and all related info nicely wrapped in a simgle xml file. Looking for a way to actually import the images I came across the Media Feeds module which basically let's you 'import' media files into as an image or file field into an entity (e.g. a node).

The module's project page (https://www.drupal.org/project/media_feeds) has also a link to a screencast – should not be too complicated once you can read the Excel table / get access to the images' urls.