Help+Manual's table formatting functions are powerful but they do have some limitations. For example, you can only set a background picture for the entire table. Merging and splitting cells in a single table can also place limitations on column width adjustments.
Some of these problems can be solved by using nested tables, i.e. by inserting a second table inside the first table. Then you can apply different table properties to the second table, which can be a single cell, row or column in the "parent" table.
Temporarily apply thick colored borders to your nested tables so that you can see where they are while you are working with them. You can do this most easily by defining a table style and applying it temporarily while you are working.
Table Properties (Reference)
How table sizing works (Reference)