You can customize your maptip template to show image from a field.You should add Image control to your template and bind the source to your filed as the following:
<esri:FeatureLayer.MapTip>
<Grid Background="Blue">
<StackPanel Margin="5">
<Image Source="{Binding [img]}" Height="150" Width="150"/>
</StackPanel >
</Grid>
</esri:FeatureLayer.MapTip>
Just make sure to add your "img" field to your featurelayer OutFields property.also you can check the following sample for more information:https://developers.arcgis.com/silverlight/sample-code/start.htm#FeatureLayerMapTips