(I'm assuming that your zipped files are indeed valid and the problem isn't in the zipping/unzipping software used.)
I'm not sure if it's a problem with FTP itself that you're witnessing here since FTP supports files larger than 7GB, but problems may arise with large files depending on the clients used to upload and download, the target file system (although probably not a problem nowadays), and the server's file system and software.
Instead, it might make sense to split the ZIP file you're transferring into chunks less than 2GB or to compress subsets of images (if applicable) into a series of ZIP files that are each <2GB. That way, if your clients are unable to resume partial downloads, they won't have to restart. If you go the split-one-big-zipfile route, you can also calculate a checksum for each chunk you upload and have your clients do the same to see which chunks have been corrupted. This can also be helpful in diagnosing where the corruption is occurring.