TRichView.com

How can I add a alt="Something to show" for an image when exporting to HTML?

e.g

<img width=208 height=208 alt="Something to show" hspace=1 vspace=1 src="img1.jpg">

is there any event like OnWriteHyperlink which I can set the Extras string?

It's not necessary to use OnWriteHyperlink for this.

There is a property of image called rvespAlt.
You can set it for the current picture:
RichViewEdit1.SetCurrentItemExtraStrProperty(rvespAlt, "Something to show", True);
This property is supported by RichViewActions (can be entered in the picture properties dialog).

PS: Alt is not actually a hint. It is a text representation of the image. Browsers display it when the picture is not available (or not downloaded yet). IE displays it as a hint, many other browsers do not. For hint, there is another property: rvespHint.