When downloading pictures even on the high speed (3mb/s) there's always many errors. Program writes everything in report. But here's the problem.
When you stop and then restart the download log rewrites. Why? Why doesn't it continue too. It would be very nice if it is so.
And here's a second thing. In full log you can check important pictures that didn't downloaded. Can you create a function to check links in the log once more and more. It's very often that links was somehow "dead" only temporary and then they are working. It would give an ability to fully download the whole site and leave only really-not-working links.
Maybe it's even a good idea to collect a history of errors to have an ability to look for links any time.