wen chu, the following method works for me.. dun ask me why though.
first go to your display settings and selected your current (existing) wallpaper again. You will find that the screen does not blacked out momentarily when you re-select the wallpaper you are using at the moment. After which, go display settings again and this time round, select the wallpaper you wish to change to. and tata... the new wallpaper is selected.
Hope that's useful. It works for
MY 7250i though. Don't know if your hp is more 'temperamental'.
Acid, the photos taken using the 7250i are quite blurry. there's really much better camera hps in the market now. maybe try the ericsson one (cant remember model no.)