Fluid Images

Hi --

I found this information about making images fluid in a previous forum post:

In the content tab you can set the container to fluid. Then just make your images 100% wide like this:

<img />

I apologize for being a dunce; I am moving from HTML upload to WP and am self-taught. I cannot find a Content Tab. I find

Content Layout, and that offers me choices between separate containers or one container.

As well, where do I find and/or place the code for img style="width%;/>? If I click on an image to edit, the drop down choice do not allow this. Is there more to this code such as image depth?