Relax detection of images for strip detection

It is perfectectly fine to have an image cited in an HTML body similar to:
```html
<img width=909 height=302 src=cid:image002.jpg@01CF5426.BF5A72A0 alt=image>
```
Which may or may not have quoted `@src` attribute, and may very well have an at sign (@) somewhere in the attribute text. The previous regular expression would not match such a `@src` attribute.