I have a vacation home and the deck is not as sturdy as I would like, I am going to install 2" X 6" diagonal bracing with 1/2" galvanized bolts below the deck but have a question. Is it better to install higher as shown on the right or low as shown on the left. I would prefer low if it doesn't matter so I can keep off a high ladder, but it higher is preferred I will install at the high section.
Can't tell from the photos, but the posts look pretty skinny to be supporting the deck. Typically a deck of such a height would be supported on 6x6 posts set on concrete sonotubes using Simpson standoff brackets. Maybe you can describe how your deck is put together.
As for the diagonal braces, they work best when they are installed from the bottom of one post diagonally upward to the top of the next post. This is rarely done, except in commercial or industrial buildings, because it looks odd. The objective is to make the diagonals as long as possible, so from halfway up one post to the top of the next is OK. Use two galvanized through bolts (typically half inch diameter) on each end of each brace (four total per brace). You can use six (three on each side), but it is not necessary.
The braces work because if the deck tries to move laterally, the triangle shape formed by the post and brace causes either tension or compression forces in the brace. The braces are very stiff and strong in resisting compression and tension forces, so the deck is unable to move much laterally, assuming the braces are rigidly bolted to the posts, which requires two or more bolts per end of the brace. The longer the brace, the more resistant to movement.