All depend of what component snippet are you trying to use and how are you trying to use it. All html components have their own limitations regarded of where it can be placed and some non documented rules of course. But with your own css and using odoo development features for the odoo website you could do anything. If you just wanna put an image exactly over another one I think that that could be do it b simply moving that specific component using the move tool that appear when you select a component that is composed by 4 arrows pointing to the edges of an invisible square. Also all of the possible css options are not covered but it is possible to add it what you want using snippet options and animations

The Odoo website builder is based on the bootstrap responsive grid framework. Depending on the website theme you have installed, you have different snippets/building blocks available. You can also customize your website using the built-in HTML editor or you can build your own snippets and layouts. More information here: https://www.odoo.com/documentation/10.0/howtos/website.html