In the email you are working on, add an Image Component either via the Add Block controls or by selecting the Image component in the right-hand pane.

Once the Image Component has been added, click on it to access the image controls in the right-hand pane.

Click the plus(+) button in Image Selection then select Content Library.

The Content Library is where you can store your frequently used assets such as logos, photos, etc and just select them for inclusion in your latest email.

In the Content Libary window, use the UPLOAD button to locate the image on your PC and then add them to the library. After the image has been added, click the button next to it, then click on the Select button at the lower-right corner of the window.

This same process works in Briefings as well.

All images in the Content Library can be accessed and managed from Account > Content Management > Content Library