Can't post full size image after changing permalink

System : WordPress 3.5.1, installed on IIS7.
Problem :
For some reasons with Weblibrarian, I have to change the permalink.
I changed it to “Numeric”

After changing it, I realized that all my previous pictures, when I click on the thumbnail, I get 500 internal server error. Images that is displayed in Full Size can’t be displayed. Thumbnails are okay though.

So, after reading here and there, I went to my root directory and changed the permission. Actually, I didn’t change anything. I just go into the folder permission, uncheck and check back, and click ok. This will trigger the computer to process changing the permission to all subfolders, although I didn’t change anything at all.

After doing this, my website works now. Those affected images are okay now.

But, now the problem is… when I add new images in posts/pages. When I add a new image and display in full size, I can’t do so (it will appear as a bad link). But if I display as thumbnail, it’s fine. But when I post the page with the thumbnail, and I click on the image, I’ll get the 500 Internal Server Error again.

Then, I would go back to my directory, change the security setting again, check and uncheck the permissions. And then, go back to my wordpress, and it will work.

What can I do? I’ve tried to remove my web.config file, but after removing it.. all my pages except home, is dead.

Gosh… this is really DRIVING ME NUTS! Desperate for help here.