The template tag {content_image} does not work well when both "id" and "block"
are set. When writing a property, the "id" field is used as key for column
"prop_name" in the table, while for reading the "block" field is used to search
on "prop_name". The result is that when both "id" and "block" are given as
parameters to the {content_image} tag, always an empty image is returned if "id"
and "block" are non-identical.

So:

    {content_image dir="images" id="img1" block="First image"}

creates a row with "prop_name" = "img1".

When reading the block content, the "block" parameter, here "First image" is
used to look for the right "prop_name", which is not found.

Everything works well when "id" is removed as a parameter:

    {content_image dir="images" block="First image"}

because then "First image" is used as "prop_name" value.