Would you perhaps be better handling that in your template rather than your model? Only I imagine you'll need a static image within the project rather than a dynamic media item for a default image.

You could also then set the default image on a template by template basis.

Another option is to override the SetDefaultValues() method perhaps, or in the back office via CMS > Admin > Content type > your item - however I'm not sure how you'd set a default value for that particular type.

There is an example on how you can accomplish fallback in your model property by implementing custom getter/setter for your property in the SiteLogotypeBlock.Url property in the Alloy template package.

I had the solution open, so I thought I'd try save you a few minutes :)

        [UIHint(UIHint.Image)]
[DefaultDragAndDropTarget]
public virtual Url Url
{
    get
    {
        var url = this.GetPropertyValue(b => b.Url);
 
        return url == null || url.IsEmpty()
                   ? new Url("/Static/gfx/logotype.png")
                   : url;
    }
    set
    {
        this.SetPropertyValue(b => b.Url, value);
    }
}

Is there a reason you'd use this rather than the SetDefaultValues? I'm still trying to resolve everything I learnt on the fundamentals course with what I've actually seen in practice.

The difference between setting a default value and doing fallback when fetching the value is that the first will save a value in the database. This means that it's not as simple to change this over time since you have the value defined in lots of different places. Also, for a use case as in the Alloy template, it feels a bit akward to save a reference to a fallback image. That of course does not mean that it's always the right choice.

Ah, I understand, so with the getter/setter method you can change you placeholder image site wide at a later date and also not grow the database. Better method then in this case :)

Exactly, so in most cases this is probably preferred before a default value if you think that there is a big chance that the editor will not change the image.