Is there any error message in your PHP error log?
Uploads should also typically be placed inside dated folders for WordPress. This means that when you upload a file it goes into the folder 2014 for the year, the date, etc. So you shouldn’t have 13k images in one folder if you’re uploading through the admin area.
Some FTP programs will restrict a listing of all files to around 2,000, but there shouldn’t be a problem adding more images like that.
I would check your PHP error log file to see if anything useful is there.
