O artigo a seguir foi retirado dos artigos do TechNet e é para trabalhar com o Windows 7, mas não acredito que haja diferenças importantes entre os arquivos xml autônomos do Win 8 e do Win 7.
< ?xml version="1.0" ?>
<unattend xmlns="urn:schemas-microsoft-com:unattend">
<settings pass="windowsPE">
<component name="Microsoft-Windows-Setup" publicKeyToken="31bf3856ad364e35" language="neutral" versionScope="nonSxS" processorArchitecture="x86">
<WindowsDeploymentServices>
<Login>
<WillShowUI>onerror</WillShowUI>
<Credentials>
<Username>wdsadmin</Username>
<Domain>myschool</Domain>
<Password>wdspassword</Password>
</Credentials>
</Login>
<ImageSelection>
<WillShowUI>onerror</WillShowUI>
<InstallImage>
<ImageName>Windows 7 Professional SP1</ImageName>
<ImageGroup>x86 Images</ImageGroup>
<Filename>Windows7proSP1.wim</Filename>
</InstallImage>
<InstallTo>
<DiskID>0</DiskID>
<PartitionID>1</PartitionID>
</InstallTo>
</ImageSelection>
</WindowsDeploymentServices>
<DiskConfiguration>
<WillShowUI>onerror</WillShowUI>
<Disk>
<CreatePartitions>
<CreatePartition>
<Order>1</Order>
<Type>Primary</Type>
<Extend>true</Extend>
</CreatePartition>
</CreatePartitions>
<ModifyPartitions>
<ModifyPartition>
<Active>true</Active>
<Extend>false</Extend>
<Format>NTFS</Format>
<Label>OS</Label>
<Letter>C</Letter>
<Order>1</Order>
<PartitionID>1</PartitionID>
</ModifyPartition>
</ModifyPartitions>
<DiskID>0</DiskID>
<WillWipeDisk>true</WillWipeDisk>
</Disk>
</DiskConfiguration>
</component>
<component name="Microsoft-Windows-International-Core-WinPE" publicKeyToken="31bf3856ad364e35" language="neutral" versionScope="nonSxS" processorArchitecture="x86">
<SetupUILanguage>
<WillShowUI>onerror</WillShowUI>
<UILanguage>en-UK</UILanguage>
</SetupUILanguage>
<UILanguage>en-UK</UILanguage>
</component>
</settings>
</unattend>
Se você olhar bem de perto, existe
<InstallImage>
<ImageName>Windows 7 Professional SP1</ImageName>
<ImageGroup>x86 Images</ImageGroup>
<Filename>Windows7proSP1.wim</Filename>
</InstallImage>
O que me leva a acreditar que o seu arquivo xml não aponta para o arquivo de imagem correto ou você não tem um.