Assuming this is EmguCV, QueryFrame returns a Mat, and calling ToImage on that returns an EmguCV Image class instance - which is not related to the .NET Image class - so it doesn't support a Bitmap property:
Image(TColor, TDepth) Class[
^] which the PictureBox class expects.
You may be able to convert it - SO seems to think you can:
https://stackoverflow.com/questions/15406235/convert-emgucv-image-to-system-drawing-bitmap[
^] - but ToBitmap isn't listed in the EmguCV Image documentation so I have no idea if it will work ...