I always have the hardest time getting a custom image header the right size in Google Forms (the “new” Google Forms, of course). I made this Google Drawing template for my use but am making it available to anyone else needing it.
Get your own copy by going to http://bit.ly/formimagetemplate. Instructions are on the sides of the drawing but here is how it works.
The image canvas is set to be 1600 x 400 pixels. There is a rectangular area that represents the center top above the form. If you put your design elements there, you will be a-ok. When you have that area the way you want it, you can delete the rectangle (unless you want to keep it visible on the form). Then, download the image as a PNG. When you upload the image into the color palette on the Google Form, set the cropping markers to match the layout of the canvas on the template.
Here is a quick video – no audio – showing the process.