Image upload with preview

I'm not sure about the preview.  But the control uploads the file to a temporary location (based on a context id)  When you process the entire form and commit the save, you read from that temporary location and push it to a file storage location (typically).

I would look at existing widgets and find one that does something similar and use that for reference.  The blog post create widget has an image upload with a preview.

There is a discussion of custom file storage providers here.  Basically you will pass in the temporary file storage info when saving, then in C# code save the file to your custom storage provider and save your object with a reference to its permanent location.