You seem to be missing one of the function sources. 

You should find a dll which contains the function `GetImageAlt` and add it to the GAC. After, you need to add a reference to this dll into the `functionSources` section of your `Tridion.ContentMananger.config`. See an example snippet of the `Tridion.ContentMananger.config` below:

<!-- language: lang-xml -->

    <tridion.templating>
      <functionSources>
        <functionSource type="Tridion.ContentManager.Templating.Expression.BuiltInFunctions" />
        <functionSource type="MyCustomClass" assembly="MyCustomClass, Version=1.0.0.0, Culture=neutral, PublicKeyToken=ddfc8957d6e5ee6b" />
      </functionSources>
    </tridion.templating>