Hi Colin
The apps you can upload to all compress your files.
Can you provide details of one of the pics ie file size, format.
What is the spec of your PC? ie Speed, memory, hard disk size. What is your operating system? Which browser are you using? Do you have any Virus protection etc?
What internet connection do you have?
There could be a whole raft of reasons why you are having problems and without more info it's difficult to give advice.
One thought, if your successful uploads are all at a particular time of the day it could be your internet speed connection.
Try running Speedtest by Ookla to check your speed
http://www.speedtest.net/
Dave