Improve image docs and standardize usage

What's wrong with the current model?

Misleading image field. In most of the example (test) files, this field is used for image URLs but in some it's used to provide image classes, for example in SCMessage.

How should the model be instead?

In the inline documentation should be written, what this field (image) is really for - for image URLs.

Why should the model be that way?

To avoid misunderstanding.