First create External Page Teaser:

• open the page demo-project / about
• in Extra, add a new component External Page Teaser
• add an URL and in the tab TeaserOverwrite add an image
• save

You must see that only the url is displayed not the image.

Why?
First:

• open the associated flt /templating-kit/components/teasers/externalPage.ftl
• open the modelClass info.magnolia.module.templatingkit.templates.components.ExternalTeaserModel

You will find in the ftl these lines:

[#if !hideTeaserImage]
    [#assign imageLink = (model.image!).link!]
[/#if]

You can try to find in the modelClass the method getImage(), but you wont find it...

Solution:

• add a method getImage to the model
• or maybe it is more easy if the model extends info.magnolia.module.templatingkit.templates.components.ImageModel