GridMarket Header Logo

I am wanting to change the default use of the header logo function within the FrameMarket/Gridmarket theme. I just want to upload a hi-res PNG/transparent background logo and not create a 980x100 image. I have tried creating the PNG format in the 980x100 dimension and it shows up black on the upload so I'm wondering where in the code I can hardcode this to show the image I want only.