Here is a link to a list of edible flowers. I have used carnations and roses on cakes before with no problems. You can find flower stakes that you push into the cake and then put the flowers in them (like mini vases) If the cake will sit out for a while, you can also add some drops of water into the stake to keep the flowers fresh.
I use only edible flowers when using real flowers just to be safe. If it touches the cake anywhere, the flower itself should be edible as well.
I usually will make a small flower bunch and use a bit of floral tape to hold them together, wrap the stems with a bit of saran wrap (since the stems are not edible) and then put them in the flower stake- just because I"m overly cautious incase a stem starts to lean and ends up touching the cake.
But that's it.
there is also a link to poisonous flowers on this page. Hyrangeas are poisonous and I wouldn't use them. In the pictures they could be gumpaste or sugar flowers that you are seeing.
I'll try to get my photos of my cakes with real flowers into my profile later today.