<img src="local-dir"> not working?

So what I am trying to do is:

1. load some small images, display them as buttons, in html
2. users may click buttons to open local image files then display them

And looks like loading images on internet or loading with base64 encoding works fine, but loading images from my own machine doesn't work. Is this a bug, or is it actually how webview works, or did I mess something up?

And if this is expected behavior of webview, I can kind of make case 1 work by base64-encode all button images and inline them into html, but that makes html awkward to maintain, and for case 2 I really have no idea how to make that happen. Any tips?