Exactly what the error message said.
You need to cast the Image to a Bitmap:
GraphicsImage = (Bitmap)Image.FromFile(openfiledialog1.FileName);
By saying that an "explicit casts exists" you're hinted that you must do a type cast. Another verions is this:
GraphicsImage = Image.FromFile(openfiledialog1.FileName) as Bitmap;
You have two options if you want to make the image fit into the panel:
1. Keeping the images aspect ratio you need to make this adjustment
panel1.BackgroundImageLayout = ImageLayout.Zoom;
2. Filling the whole panel with the image disregarding the aspect ratio of the image
panel1.BackgroundImageLayout = ImageLayout.Stretch;
Solution 2 will leave you with a distorted image if the panel's dimensions don't match the dimensions of the image.