Ok! Yeah, the whole content pipeline thing can be tricky for a lot of us to fully understand 
Luckily, once everything is setup, and the workflow is established, it will run smooth, no problems.
I think you should delete those png files from your project enirely (including the actual files in the build dir, any xnb files by the same names)…
Then, add the png files to your project simply by dragging the files into your content folder in the solution explorer side-bar.
Then high-light all these new png items in the solution explorer, and set their properties fields to ‘Content’ and ‘Copy if newer’…
You don’t really need the tool for png files in the first place, it just adds another layer of confusion