What about a throw away IEqualityComparer generic class ?

public class ThrowAwayEqualityComparer<T> : IEqualityComparer<T>  {    Func<T, T, bool> comparer;      public ThrowAwayEqualityComparer<T>(Func<T, T, bool> comparer)    {      this.comparer = comparer;    }      public bool Equals(T a, T b)    {      return comparer(a, b);    }      public int GetHashCode(T a)    {      return a.GetHashCode();    }  }
So now you can use Distinct.

var distinctImages = allImages.Distinct(     new ThrowAwayEqualityComparer<GalleryImage>((a, b) => a.Key == b.Key));
You might be able to get away with the <GalleryImage>, but I'm not sure if the compiler could infer the type (don't have access to it right now.)

And in an additional extension method:

public static class IEnumerableExtensions  {    public static IEnumerable<TValue> Distinct<TValue>(this IEnumerable<TValue> @this, Func<TValue, TValue, bool> comparer)    {      return @this.Distinct(new ThrowAwayEqualityComparer<TValue>(comparer);    }      private class ThrowAwayEqualityComparer...  }

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