byte[] binaryData = Convert.FromBase64String(bgImage64);

BitmapImage bi = new BitmapImage();
bi.BeginInit();
bi.StreamSource = new MemoryStream(binaryData);
bi.EndInit();

Image img = new Image();
img.Source = bi;

扩展Josh上面的答案-这里有一个使用ValueConverter的方法，这样您就可以使用绑定，而不是在代码隐藏中设置图像源。
Base64ImageConverter.cs

public class Base64ImageConverter:IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        string s = value as string;

        if (s == null)
            return null;

        BitmapImage bi = new BitmapImage();

        bi.BeginInit();
        bi.StreamSource = new MemoryStream(System.Convert.FromBase64String(s));
        bi.EndInit();

        return bi;
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

ImageData.cs（用作数据源）

public class ImageData
{
    public string Base64ImageData { get; set; }

    public ImageData()
    {
        this.Base64ImageData = "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";
    }
}

Window1.xaml

<Window x:Class="ImageFromData.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:ImageFromData"
    Title="Window1" Height="300" Width="300">
    <Window.Resources>
        <local:Base64ImageConverter x:Key="Base64ImageConverter"/>
        <local:ImageData x:Key="ImageData"/>
    </Window.Resources>
    <Grid DataContext="{StaticResource ImageData}">
        <Image Source="{Binding Base64ImageData, Converter={StaticResource Base64ImageConverter}}"/>
    </Grid>
</Window>

(root项目的命名空间为ImageFromData）

在我遇到的一个情况下，我创建了一个转换器来处理Base64字符串到WPF图像的转换。因此，您可以将它绑定到您的base64字符串属性，并让转换器处理其余的工作。公认答案和我自己的答案之间的主要区别在于，MemoryStream是一个IDisposable资源，应该使用try/catch/最后，在使用完资源后，释放资源，或者使用“using”语句确保在垃圾收集器中正确捕获所有内容。否则，您可能会遇到内存泄漏，或者如果在较大的一侧有足够的图像，您可能会遇到“OutOfMemory”异常。
https://learn.microsoft.com/en-us/dotnet/api/system.io.memorystream?view=net-6.0
https://michaelscodingspot.com/find-fix-and-avoid-memory-leaks-in-c-net-8-best-practices/

public class ByteToImageConverter : IValueConverter
    {
        public object Convert(object value, Type targetType,
            object parameter, System.Globalization.CultureInfo culture)
        {
            if (string.IsNullOrEmpty(value?.ToString()))
                return null;

            var imgBytes = System.Convert
                .FromBase64String(value.ToString() ?? throw new InvalidOperationException());

            using var stream = new MemoryStream(imgBytes);
            return BitmapFrame.Create(stream,
                BitmapCreateOptions.None, BitmapCacheOption.OnLoad);
        }

        public object ConvertBack(object value, Type targetType, object parameter,
            System.Globalization.CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }