On my old site, I had a few hundred user uploaded images. I've FTP's these to the new silerstripe site, and manually added the links in to the 'Files' table in the database, and another dataobject table which links the photos to a gallery
This works fine if the image is in the main /assets/Uploads/ folder, but the problem is all my old galleries were in individual folders,
basically it seems silverstripe just isn't creating the resampled images that are in the folders?
I've had some issues when uploading files through the FTP as well. If the dataobjects are currently referencing the correct files check the permissions on the directories. My FTP user account uploads files with 000 permission. I've gone so far as to manually create the _resampled folder in my subfolders and ensure permission is 766 or 777 on both the resampled and its parent folder.
On a linux system it comes down to the permission groups your FTP user and Silverstripe are running in.
But like I said first ensure your DataObjects are referencing the correct files