You don't need to do anything. The "error" simply means that at some point the search engine asked for the directory by name and very properly received a 403 because there's no index.
400-class "errors" listed in wmt don't always mean that you've done something wrong. It's just for your information.
Once a search engine knows of a directory's existence-- for example by seeing named files within the directory-- it's reasonable for it to try requesting the directory itself. I haven't personally seen it with image directories, but it's routine for anything that contains pages but no index file.
wmt should have a column showing where there erring file was linked from.
if you click the 403 url in the "Crawl Errors"/"Access denied" list in GWT, a window will pop up with information about that url. examine information in the "In Sitemaps" and "Linked from" tabs for clues.
as lucy24 mentioned google may not have discovered this actual url as google is known to engage in "discovery by url hacking".
Thanks lucy24 and phranque, sound like i don't need to do anything then.
Please explain about 404 error seeing in GWT also I have removed few URLs which I no longer needed are responding 404 and submitted URL removal request in GWT. These URLs are appear in Crawl Error section now, do I need to "Mark as Fix" or leave it there. How long usually does it take to remove from google index. Thanks
A 404 error in wmt is for your information, like a 403 error. It simply means "google tried to crawl this page and couldn't find it".
If you removed the page on purpose then 404 is appropriate. But if you're sure it's not coming back you might prefer to code a 410. The googlebot will then go away much faster, though other search engines might not. Be sure you have a nice custom 410 page. It can even be the same physical page as your custom 404 page; this is just for humans.
"Mark as fixed" simply means "Yes, yes, I know, stop bugging me about it." As far as I know it doesn't send any information to google; it's only for your own convenience.
I've never actually timed it, but removal requests are fairly instant and should definitely take less than 24 hours. You'll see the request status change from "pending" to "removed".