problem with header image

Hi there
I have a problem with a very recently (version 1.0.6) version of Agama Blue. Instead of showing the nice lizard picture it shows the picture that appears as the default in Agama the parent theme. Also with the animation/twizzly bits flying around overlaid on the image.
I would like to replace the default image with my own headers. I have uploaded headers (choosing a different size from 1920 x 960, which is what is suggested by the theme documentation). I have randomized the headers. That all works fine – the new images appear, they randomize, etc but below them still appears the non-lizard picture covered in the animation. When I go and switch the animation off the animation stops appearing over my own header images but still appears over the default image.
I can’t find the default image in the list of header images that appear in the customizer (only my own uploaded ones) so I don’t know how to delete/remove it.
I suspect this problem may be caused by the fact that my images aren’t the exact same size as the default/recommended image. However, I require the images to be a different size (not as high) so that the various boxes that appear under the header image appear higher up the screen when viewed on a laptop screen.
I would be really grateful for some advice – thanks in advance for the time spent on considering this!
Cheers
Liz