Using ImageField to add an inline image (save thumbnail, and when clicked open with colorbox)
If you enable core File and Image modules you can add image upload fields to different nodes/entities and create image styles (to get thumbnails). The number of allowed image uploads can also be set. The image uri will be available in the node.tpl.php file which means you will have full control over how it appears. Here's example code showing about how it could look like in the node.tpl.php file (with colorbox module 'Colorbox load' enabled - alternatively just use javascript):

&lt;?php if (!empty($node-&gt;field_image['und'][0]['uri'])): ?&gt;
  &lt;?php
  $full_img_url = file_create_url($node-&gt;field_image['und'][0]['uri']);
  $thumbnail_url = image_style_url('thumbnail_style', $node-&gt;field_image['und'][0]['uri']);
  ?&gt;
  &lt;a class="colorbox-load" href="&lt;?php print $full_img_url; ?&gt;"&gt;&lt;img src="&lt;?php print $thumbnail_url; ?&gt;" alt="Image" /&gt;&lt;/a&gt;
&lt;?php endif; ?&gt;


The code is not tested so there might be mistakes, but it should still be helpful. You'll have to set the image field hidden via 'manage display' or unset/hide it in the template file otherwise you'll get a duplicate printed with $content.