Image tag fixup makes some potentially inconsistent assumptions about paths

Looks like this:

```python
image_url = (os.path.splitext(src[1:])[0] + "/").lower()
image_document = app.jinja_env.site.post(image_url)
```

Should probably look like this:

```python
image_url = page.url_for_path(src[1:])
image_document = page.post(image_url)
```