Question: I want to change the background of a grid containing an image and a textblock dynamicly with a specific duration and a nice animation (like tiles for exemple), let's say every 30 seconds, from a flux XML. any help please ?

Answer: 
Use tile control from [coding4fun](http://coding4fun.codeplex.com/). See [here](http://www.windowsphonegeek.com/articles/Getting-Started-with-the-Coding4Fun-toolkit-Tile-Control)

For dynamic update:
1. Create a dispatcher timer and update the tile property in callback
(Or)
2. Bind tile background dependency property to Image source property in a viewmodel. As long as you figure a way to update viewmodel, the ui will get refreshed.
Have-you try one method? have you search on the web ? 

For tiles effect (and more effects...) you want use [Telerik Rad control](http://www.telerik.com/products/windows-phone.aspx) (animation named [HubTile](http://www.telerik.com/products/windows-phone/overview/all-controls.aspx#hubtile)) but isn't free.. you want to try the trial version :)