There are two (2) side to the Content Manager: Template and Custom.
You can only save objects to the content manager under the Custom Tab.
The Template tab is a protected area where all the purchased and provided objects are available but you can not overwrite them.
Select the Custom tab and the area you want to save to, the "Plus" button should then be lit up and you can click it to save the selected object to that area.
Using the Find File to locate a folder is not really recommended but the real problem is that you can not use a png file directly as a prop.
You must first drag or import your image file to the stage, select the type of object you want to create (i.e. Prop) and then save it in the Custom area.
Dropping or importing the file to the stage coverts your image to an internal ctprop format that can be used by CTA.
You can use the Find File function to rearrange directories and files, create new directories, etc. if needed but those functions is also available from the content manager (for the most part).
See my CTA Tutorials on the YouTube channel CTAStepByStep
Check out EZColors for CTA. The easy way to change Render Styled (RS) colors.