Just A Bit

А документацию придумали для кого? Всё же описано... https://dev.mysql.com/doc/refman/5.7/en/precision-...

The declaration syntax for a DECIMAL column is DECIMAL(M,D). The ranges of values for the arguments are as follows:

M is the maximum number of digits (the precision). It has a range of 1 to 65.

D is the number of digits to the right of the decimal point (the scale). It has a range of 0 to 30 and must be no larger than M.

Если 1 статья + 1 картинка (только надо выбрать нужный "tmplvarid")

SELECT * FROM modx_site_content AS msc
LEFT JOIN modx_site_tmplvar_contentvalues AS mstc
ON msc.id = mstc.contentid AND mstc.tmplvarid = 1
ORDER BY mstc.contentid DESC

Все картинки + Все статьи к ним

SELECT * FROM modx_site_tmplvar_contentvalues AS mstc
LEFT JOIN modx_site_content AS msc
ON msc.id = mstc.contentid
ORDER BY mstc.contentid DESC

LEFT JOIN или INNER JOIN, решайте сами.

Надеюсь я вас правильно понял :)