that's really more of a css issue than that of concrete5.
say your have this:

<div id="banner></div>
<img id="logo" src="logo.png" />

then you could position the png on top by having a common container around them like

<div id="bannerContainer">
<div id="banner></div>
<img id="logo" src="logo.png" />
</div>

then use the css properties controls positioning like so:

#bannerContainer { position: relative; }
#logo {position: absolute; left: 200px; top:10px; z-index: 10;}

saying #logo is absolutely positioned within the bounds the the closest parent element that's either relative or absolute in position (the #bannerContainer in this case).

concrete5-wise, maybe try using the "design" option for the image block for instance if it's placed below the banner/slider. apply position:relative and top:-100px and see what happens.