Amazon Drive for Photos
Amazon does not care about the folder structure, it only cares about the file type
This is why I started using CB for this. With the Amazon App, it only allows one folder to be synced. I have pics in different folders and didn't want to have to restructure everything. If I would have synced the next folder up, it would have included other folders with other file types (that aren't unlimited).
With CB, you can choose whatever folders you want. You can also filter out files that aren't pics if you have multiple file types in one folder.
For me, this works great. I have about 100GB of pics in there from multiple machines.