As you have put your image in theme web folder public_html/app/design/frontend/Infortis/ultimo/web/ so you have to use
instead of $this->getViewFileUrl('Infortis_Ultimo::images/membership-banner.jpg').
When you have use Infortis_Ultimo:: it indicated that folder is public_html/app/design/frontend/...
I'm not sure I get exactly what your asking.
If the structure is right in your extension and you di.xml file is correct.
Once you run:
Your extension will be installed.
No need to merge the dl.xml file manually.
If its beyound installing a extension. I can't help much.