how can I change my garden owner pic please and reason for my garden ?
Your garden owner pic gets pulled from Facebook so you would need to change your profile picture there. If you want to change your garden description go to "about this garden" and click on the edit description button :)
Click on red mushroom upper left corner....then you can click to change garden status, name or picture
It's not under the mushroom (I know; everything else is!). The garden status, name and picture are changed from the right hand side of the garden page, under About This Garden, as Peta said.
Garden status and backwall photo (which also shows next to FB person photo on front wall above the larger garden photo) is changed from mushroom (also fairy name is changed from mushroom) Garden name and description and front wall garden pic are changed from right side, true.
It's all the fairy's information under the mushroom - you can change her status, picture and name there, but the stuff about the garden is on the garden page :)
And of course as Peta also said, the garden owner's picture is the Facebook profile picture, so that can't be changed from within the game at all.
(and I assume that by "reason for the garden" the OP meant her garden status rather than the fairy's status).