image url Syntax is ok for both static block and phtml, But make sure that when you write this code in static block

<span class="tag">&lt;img</span> <span class="atn">src</span><span class="pun">=</span><span class="atv">"{{view url="</span><span class="atn">images</span><span class="pun">/</span><span class="atn">slide-bg</span><span class="pln">.</span><span class="atn">jpg</span><span class="atv">"}}"</span> <span class="atn">alt</span><span class="pun">=</span><span class="atv">"test"</span> <span class="tag">/&gt;</span>

on front end it looks like

http://localhost/magento2new/pub/static/frontend/Magento/luma/en_US/images/slide-bg.jpg

Where Magento/luma is package name and theme name, you can replace with your package and theme name

So you have to take care for slide-bg.jpg is exist in

pub/static/frontend/Magento/luma/en_US/images directory.

In your case,

<span class="pln">&lt;img src='</span><span class="pun">&lt;?</span><span class="pln">php echo $this</span><span class="pun">-&gt;</span><span class="pln">getViewFileUrl</span><span class="pun">(</span><span class="str">'images/footer-logo.png'</span><span class="pun">);</span> <span class="pun">?&gt;</span><span class="pln">' alt="Demo"&gt;</span>

Syntax seems ok , but in frontend It looks like

http://localhost/magento2new/pub/static/frontend/Magento/luma/en_US/images/footer-logo.png

So you have to make sure that footer-logo.png exist in

pub/static/frontend/Magento/luma/en_US/images directory.

And don’t forgot about "[quotation marks]

In CSS:

background: url('{{view url="images/menu-button-of-three-lines.svg"}}');