Binding image source dynamically on xamarin forms

You said:

the image is in "Images" folder on portable project

Each platform have a different approach for resources and images. Xamarin handles that in every platform for example Xamarin.iOS has embedded resource while Xamarin.Android uses Android resource for images.

You should place your images in every project for Xamarin.Forms to know where to look for them in each platform.

For more information look at this.

I'll post this answer here because it's the one I was looking for and I suspect most others are (and Google currently directs you here, which currently isn't super helpful).

How to bind an image using XAML:

XAML:

<Image>
    <Image.Source>
        <FileImageSource File="{Binding SomeImage}" />
    </Image.Source>
</Image>

Then in the ViewModel, C#:

public string SomeImage
{
    get { return string.Format("prefix-{0}-suffix.png", OtherProperty); }
}

Newer, better, but essentially equivalent c# you can use instead of the above:

public string SomeImage => $"prefix-{OtherProperty}-suffix.png";

This is certainly the easiest way to set it up, IMO :).

EDIT: It should go without saying, but the image should obviously be in the Resources folder of the project for each platform.

EDIT2, much later: In case it's not obvious, "prefix", "suffix" are just random strings, SomeImage just has to return the path of your image. SomeImage and OtherProperty are members of your view model class, OtherProperty is just something you're basing your image name on (because if you know the whole name in advance, you don't need this question).