How to check if a datafield does contain a picture (Bmp) or is empty?

When loading my form, I need to adjust the size of the picture to the size of the TdbImage on the form.
But when I try to do that on a empty datafield it gives an errormessage.

So I have to check if the datafield does contain a picture (Bmp) or is currently empty before calling the resize procedure?
How can I do that?

Thanks Stef