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There are several ways to do it.
What version of Mac OS X are you using ? Click on the Apple icon and select About This Mac. You can find the version number there
Go to this website and download and install Dropbox for Mac --> https://www.dropbox.com/downloading?os=mac
(If you don't see downloaded Dropbox installation files, then click on Finder icon on dock, then click on Go menu at the top bar and select Downloads. This will open Downloads. In Downloads, you will see a file named DropboxInstaller.dmg. Open it and install it)
After installing Dropbpx, click on Dropbox icon icon in the menu and sign in to your Dropbox account. Then click on Dropbox icon at top menu bar again and click Preferences. Click on Import tab and click "Import Photos from iPhoto".
Then from resulting window, click Import Photos to Dropbox
This will import photos your Dropbox including all albums in correct order. Refer this article --> https://www.dropbox.com/en/help/1963
This process is only for iPhoto and not for latest Photos app. If you are using Photos app, please let me know. Also, if you would like to export selected photos from iPhoto and import to Dropbox, then there is a different method to do it. Let me know if that is what you are trying to do.
If you have any difficulty on following the steps above, please let me know and I will continue to help you.