Replacing background graphics of a PowerPoint theme

I'm using this theme (this is my .pptx file).

When I check the "Hide Background Graphics" checkbox, the butterflies' graphics disappear.

I want to replace such buttefly graphic by my own designed one, how can I do that?

I had the same issue. To edit the background:

1. Click on View -> Slide Master
2. Click on View -> Slide Master again.

All the background images will become editable and copy-pasteable.

Definitely not intuitive.

In this case go to your Background Style and click on format background where there is a option picture/texture fill. Click on it and you can find the insert picture option. I think you can upload your desired background picture from here.