Awesome, and that's interesting to know.
Basically, this stems from me playing around with Monogame on a Mac a while back, but finding some issues with Mouse input with the OpenTK implementation that it's currently using. (The issue was that when holding a mouse button, it would no-longer update the mouse position until the button is released.)
So, with the knowledge that OpenTK is being replaced with SDL2 in the 3.6 Milestone, I decided to leave it for a while in hope that 3.6 would be released soon...
Fast-forward a few months and sadly that still hasn't yet been released. I've got a Windows machine now and decided to just try to play with Monogame anyway. I thought that if I used the Development build, that It might have the SDL implementation in place in some manner at least.
However, I noticed that it seemed unable to find any of my texture assets, throwing ContentNotFound Exceptions, and I noticed in the error text that it was looking for XNB files and not the PNGs, which is why I wanted to try and make use of the Content Pipeline utility.
Due to the length of time between looking at Monogame 3.5 on my Mac and the development build on my Windows machine, I did not connect the dots that it might be an issue with using the dev build instead of the stable one.
In Conclusion, and TL:DR;
I cant seem to load PNG texture assets when using the MonoGame development build, as it seems to exclusively look for XNB files. But I can just make use of 3.5 stable for now, so it's ok.
The original FreeImage.dll issue is still present, but thanks to @monopalle's suggestion, I can work around it, easily enough.