The goldfish.png image file was initially attached to the goldfish.ditamap file.
The Animals DITA map already has the pufferfish.pdf file attached to it in its two instances, Novice and Intermediate.
To add the goldfish.png image file to the Animals DITA map in its Novice instance, use its khubId (retrieved from the resource's URL in the Viewer page) as follows:
<?xml version='1.0' encoding='utf-8'?>
<controlFile>
<resources>
<resource>
<filePath>_animals.ditamap</filePath>
<instances>
<instance>
<filePath>Novice.ditaval</filePath>
<attachments>
<attachment>
<khubId>vQlcqgYveKfDv53M5zn82Q</khubId>
<displayName>Amazing Goldfish</displayName>
</attachment>
</attachments>
</instance>
</instances>
</resource>
</resources>
</controlFile>