It takes a long time to look at the pictures on the server while uploading the pictures. What could be the problem?
http://imgur.com/a/1sT3W

It could related to a multitude of reasons.
Are you experiencing heavy traffic?
How many images are there present in the folder?

Thank you for your answer. 20 folders. There are close to 500 files. There's no problem with traffic. Would it be faster if I split pictures into more folders?

It might (and should) be, yes.
500 files are not that many though, is the same procedure slow when you are opening that folder with your FTP client?