UG: fix image list

It's a copy of the image list from the release notes instead of a
table.

With the refactor of the user guide underway, it will be later replaced
with a generated list.

Signed-off-by: Lucas Dutra Nunes <ldnunes@ossystems.com.br>
UG: remove all compile warnings

Signed-off-by: Lucas Dutra Nunes <ldnunes@ossystems.com.br>