This seems fairly simple, I'll take it. But do take note that sprites should be drawn in a way they are facing the angle 0, which is to the right. You drew your tree shadow diagonal. Just draw it straight up, then turn by 90 (the image editor allows that) and there we go
Edit: actually, just take 'em as they are and turn the image by 45 degrees. Easy fix
Edit2: Done! Bit hacky, but easily faked. However, those are trees, which are taller than Dudsley, so their shadows would have to be have to be drawn indefinitely with Dudley standing at their base - onto other trees, for example. So, let's assume Dudley is flying around on the broom he stole from Harry!
Here is the actual file, since the forum won't let me attach them.