why do I receive wrong Notice: Undefined index?
I think that the name views-view-fields--activities--block.tpl.php is a suggestion for all blocks of the view activities - to get better specified template name check template information in that view. ('Information' link under Other -> Theme in the view builder) I think that the template name should be something like views-view-fields--activities--[block_id].tpl.php.

Change the block machine name if necessary.

Also, its a good practice to utilise this in templates to prevent such warnings:

&lt;?php if (!empty($fields['field_activities_image'])):?&gt;
  &lt;div class="image"&gt;&lt;?php print $fields['field_activities_image']-&gt;content;?&gt;&lt;/div&gt;
&lt;?php endif; ?&gt;