<$list filter="[all[current]addsuffix[/Picture]is[tiddler]]">
  <$image source=<<currentTiddler>> width=300/>
</$list>

Notice that there's no need to use Conditional Expressions to ensure nothing is displayed if the image isn't available – if the picture tiddler doesn't exist, there will be zero results for the filter, and the $list widget will never render anything.