Hi Pierre Karlsson,
To change the image in future please follow the steps which is provided below:
Goto Drashboard=>Appearance=>widget=>and (add an image) in widget-banner wide.
For the second question:
The screen shot which is provided by you looks similar buttons icons but also you may change by putting following css:
.so-widget-sow-button-wire-56184aa9ee9b .ow-button-base a{
padding:2em ;
border-radius:50% ;
}
.so-widget-sow-button-wire-08b582321822 .ow-button-base a {
padding: 1.8em;
border-radius: 50%;
}
Note: Please use the custom css plugin or child theme to write the css.
Thanks