If I set property in my user control like this

/////////////////////////////////////
private Bitmap im;

public Bitmap IM
{
   get
   {
       return im;
   }
   set
   {
       im= value;
   }
}
/////////////////////////////////////

everything works fine, but if I set it like this:

/////////////////////////////////////
private Bitmap[] im;

public Bitmap[] IM
{
   get
   {
       return im;
   }
   set
   {
       im= value;
   }
}
/////////////////////////////////////

I'm getting error when i try to add new bitmap to collection trough property
window:
Parameter is not valid.

can someone help, I dont want to limit number of bitmaps to user and I dont
want to initialize them on control start.

Thank you.