loading bitmap locks file until dispose?

[sambo]

I am loading a bitmap from a file and using it as the background image of my form.

string sBitmapFullPath = @"C:\picture.bmp";
Bitmap bm = new Bitmap(sBitmapFullPath);

Color col = bm.GetPixel(1, 1);

this.Width = bm.Width;
this.Height = bm.Height;

this.BackgroundImage = bm;

At some point later I try to modify the file C:\picture.bmp, but the file is locked. After I call Bitmap.Dispose, I can modify the file. Is there any way that I can modify the file while it is still locked? I tried to use Bitmap.Clone image, but that didn't work either. Any ideas?

Basically, I want to change the image used for my form background, but I want it to use the same file path.