We can use LaTeX source from another WYSIWYG LaTeX editor and copy-paste into blog or Forum in HTML mode. Follow the steps bellow:

Because we use external WYSIWYG equation editor, then it is not embedded in the posting editor, we should open the equation editor and the entry blog/posting editor in the different windows/tabs. Then follow these steps:
1. Open WYSIWYG Equation Editor in a new window/tab.


2. Create an equation by click the symbl or samples of equation

3. Click Copy to Document, the the windows will directly change to the page where the WYSIWYG equation editor is.
4. Copy the code from the box by blocking and copying
5. Paste the code to the posting editor either in Compose or Edit HTML mode.

6. Then you change to Rich Text mode again, and you will see:

7. If you come back to HTML editor and want to change the equation, you will see the script has been changed. It is recommended to repeat the steps 1-6.

LaTeX code for Advanced User 

Basically all the code can be used in a posting . We can read Short Math Guide for LATEX  for looking up the type set.Here some example codes:

\frac{{\displaystyle\sum\nolimits_{n> 0} z^n}}{{\displaystyle\prod\nolimits_{1\leq k\leq n} (1-q^k)}}

\[\frac{{\displaystyle\sum\nolimits_{n> 0} z^n}}{{\displaystyle\prod\nolimits_{1\leq k\leq n} (1-q^k)}}\]

V = \frac{k_2{[E]}+{[S]}}{K_m + {[S]}}}
\[V = \frac{k_2{[E]}+{[S]}}{K_m + {[S]}}\]

6CO_2 + 6H_2O \xrightarrow{Light Energy} C_6H_{12}O_6 + CO_2 \ \delta G^{\circ} = +2870kJ/mol
\[6CO_2 + 6H_2O \xrightarrow{Light Energy} C_6H_{12}O_6 + CO_2 \ \delta G^{\circ} = +2870kJ/mol\]



Sources: