The media picker field is not displayed in the Summary view by default. See this response to a similar question:
How to add Image Field to Blog Post Summary in Orchard CMS?
In short you need to add the following to your placement.info file:
<Match DisplayType="Summary">
<Place Fields_MediaPicker="Content:1"/>
</Match>
...and to display in admin list, include in the admin theme placement file:
<Match DisplayType="SummaryAdmin">
<Place Fields_MediaPicker="Content:1"/>
</Match>
Orchard's docs on placement: http://docs.orchardproject.net/Documentation/Understanding-placement-info