Producing triangles of asterisks based on height
You should read Naming guidelines and apply them to your code.
pyramid should be a local variable. Otherwise calling makeTriangle more than once will produce incorrect results.
There is no need for biggestOddNumber to be public.
singleElement should be a constant field.
You can use String.PadRight and String.PadLeft methods to pad the string with spaces (or any other character) from both sides. Instead of manually doing it in hard-to-understand loop.