The documentation for what you are trying to do is here: http://msdn.microsoft.com/en-us/library/ms646839(v=vs.85).aspx
The string needs to be formatted using nulls as delimiters for the different sections of a filter. So, your string should be formatted as follows:
openFileDialog1->Filter = "Image Files (*.png, *.jpg)\0*.png;*.jpg\0";