gnosis_omeka,
I think Will meant that if you have multiple files associated with any one item (a single row in your CSV) then you would need an additional column for the additional files (so, if some of your items contain 2 or more files - not counting thumbs - then you would need 2 or more columns containing the locations of each).
In my experience, ImageMagick has always successfully created the different resizes so you don't need to import them separately.
And, yes, you can use DropBox (or, I think, any other web directory that is accessible) to temporarily hold your files for the import. Either way, you'll need to make sure that the CSV plug-in has permissions to access that location and to write to the archive>>files directory. Otherwise you will get an error.
If you use Dropbox, your file column(s) should list the location like this:
http://youromekasite.org/plugins/Dropbox/files/yourfile.jpg
I just did this yesterday and it worked great, but if I got something wrong here, I'm sure Will or someone else will help you out. Good luck.