Hi,

1/ You can change sidebar for special category by go to Posts > Categories > edit category > scroll down and you can see options for change to another sidebar there.

2/ You can use widget "Custom HTML" and put a HTML like that to content of that widget:

\
<a href="banner-url-put-here"><img src="banner-url-image-put-here" alt="Banner" /></a>

Note:

- Change 'banner-url-put-here'  to the link you want go to when users click on banner - include http:// or https:// to this url

- Change 'banner-url-image-put-here' to the banner image url

Best Regards,

PenciDesign