Put Gardening Direct and nemesia in to your browser, there is a list of plants for different growing conditions sunny, windy etc etc and the months when you might expect to see the flowers. have a look there. Also have a look on the A-Z at the foot of the page. Click on B and then Back Garden ideas. Also S and then on seasonal gardens. You will find a garden by Four Seasons, one of our members. Her daughters wedding in their garden last year. You will be unlikely to achieve that standard in 2 years but there are lots of ideas you could use. If you then come back and tell us what your site conditions are and what you fancy growing we might be better able to assist you. It sounds like a very exciting project and 2 years is not so very long. NB. Nemesia is an annual and will need to be planted every year. Perrenials grow and die down each winter but reappear in subsequent years. Also look at shrubs. There are flowering shrubs but there is also a wide variety of foliage which will enhance the wedding photos.

If your perennials haven't grown big enough for the effect you want in time for the wedding it is quite easy to fill in the gaps with annuals. Calendulas are easy to grow and very bright, Cosmos is lovely, Californian poppies are easy and will fill quite a large area (and self seed for years afterwards, so you could sow some this year and get ahead!) Don't forget to have some containers too - you could spend some time this summer looking at other peoples tubs for ideas. When you are selecting plants remember to check the flowering times, so they will look their best on the Day.
Hardy perennial geraniums are reliable, especially for semi shaded areas, and make a good foil for your taller plants.