Is it possible to add inline image?

> Please provide us with the following information:
> ---------------------------------------------------------------

### This issue is for a: (mark with an `x`)
```
- [ ] bug report -> please search issues before submitting
- [x] feature request
- [ ] documentation issue or request
- [ ] regression (a behavior that used to work and stopped in a new release)
```

### Minimal steps to reproduce
> none

### Any log messages given by the failure
> none

### Expected/desired behavior
> For my case, I want to send an email with header/footer, both are inline images, I did not find any api to support it, is it possible to do so?

### OS and Version?
> Windows 7, 8 or 10. Linux (which distribution). macOS (Yosemite? El Capitan? Sierra?)

### Versions
> any

### Mention any other details that might be useful

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