If you are using K2 and wanted to have a default Item image, even without uploading one, follow below steps...

Open item.php file and find below code (Line198)

		<?php if($this->item->params->get('itemImage') && !empty($this->item->image)): ?>
		<!-- Item Image -->
		<div class="itemImageBlock">
			<span class="itemImage">
				<a data-k2-modal="image" href="<?php echo $this->item->imageXLarge; ?>" title="<?php echo JText::_('K2_CLICK_TO_PREVIEW_IMAGE'); ?>">
					<img src="<?php echo $this->item->image; ?>" alt="<?php if(!empty($this->item->image_caption)) echo K2HelperUtilities::cleanHtml($this->item->image_caption); else echo K2HelperUtilities::cleanHtml($this->item->title); ?>" style="width:<?php echo $this->item->imageWidth; ?>px; height:auto;" />
				</a>
			</span>

			<?php if($this->item->params->get('itemImageMainCaption') && !empty($this->item->image_caption)): ?>
			<!-- Image caption -->
			<span class="itemImageCaption"><?php echo $this->item->image_caption; ?></span>
			<?php endif; ?>

			<?php if($this->item->params->get('itemImageMainCredits') && !empty($this->item->image_credits)): ?>
			<!-- Image credits -->
			<span class="itemImageCredits"><?php echo $this->item->image_credits; ?></span>
			<?php endif; ?>

			<div class="clr"></div>
		</div>
		<?php endif; ?>

Replace with

<?php if(!empty($this->item->image)): ?>    

<img src="<?php echo $this->item->image; ?>" alt="<?php if(!empty($this->item->image_caption)) echo K2HelperUtilities::cleanHtml($this->item->image_caption); else echo K2HelperUtilities::cleanHtml($this->item->title); ?>" style="width:<?php echo $this->item->imageWidth; ?>px; height:auto;" />

<?php else: ?>

<img src="http://placehold.it/700x350" alt="<?php if(!empty($this->item->image_caption)) echo K2HelperUtilities::cleanHtml($this->item->image_caption); else echo K2HelperUtilities::cleanHtml($this->item->title); ?>" style="width:<?php echo $this->item->imageWidth; ?>px; height:auto;" />

<?php endif; ?>