Adding a Universal Windows Platform project to an existing Xamarin.Forms Android/iOS solution is meant to "just work". However when doing this myself I have found a few things that don't work as expected, one of those was images and image paths.
On iOS and Android there are special folders where the platform stores it's image files, iOS has Resources\Images.xcassets and Android has Resources\drawable.
So on either of these 2 platforms all you need to do to add an image in xaml is something like this: <Image Source="MyIcon" /> and it would pull an image from the relevant folder for you, you don't even have to put in the file extension.
iOS - Place images in the Resources folder with Build Action: BundleResource. Retina versions of the image should also be supplied - two and three times the resolution with a @2x or @3x suffixes on the filename before the file extension (eg. firstname.lastname@example.org).
Android - Place images in the Resources/drawable directory with Build Action: AndroidResource. High- and low-DPI versions of an image can also be supplied (in appropriately named Resources subdirectories such as drawable-ldpi, drawable-hdpi, and drawable-xhdpi).
Windows Phone - Place images in the application's root directory with Build Action: Content.
Windows/UWP - Place images in the application's root directory with Build Action: Content.
The project I am working on has just over 200 images, so putting them in the project root is ridiculous!
I created a custom renderer for the Image control on UWP and from there I could override the native image source and change the path of the image to one that suited me.
This isn't a perfect solution but it's done the job so far, as long as all your images are in the same folder and have the png extension.