Can you define "custom banner"? An image?
If so, 3 options I know of:
2) You can also use Custom Shapes in Tableau to utilize Images. https://www.tableausoftware.com/public/blog/2013/10/creating-and-utilizing-custom-shapes-2171
3) Use images that are hosted on the web, via URL.
A fourth option is a Background Image, but not sure that applies here.
I tried putting image on top of dashboard. but when you publish it you can still see the tableau logo and workbook name and login name and under that the custom banner image will come is there a way I can replace tableau logo ?
Similarly, if you are just sending out the URL, if you just append:
to the end of your URL, you should get rid of all the "excess" that you are looking to remove.