Using theme_get_setting getting empty value
Change the type of image input to managed_file. Check updated code below:
$form['mythemename_settings']['loaderImage'] = array(
  '#type' =&gt; 'fieldset',
  '#title' =&gt; t('Loader Icon'),
  '#collapsible' =&gt; TRUE,
  '#collapsed' =&gt; FALSE,
);

$form['mythemename_settings']['loaderImage']['loader_img'] = array(
  '#type' =&gt; 'managed_file',
  '#title' =&gt; t('Upload Loader Icon Image'),
  '#default_value' =&gt; theme_get_setting('loader_img','mythemename'),
  '#description' =&gt; t(&quot;Upload your desired loader icon.&quot;),
  '#upload_location' =&gt; 'public://images/',
);

Values of theme settings are stored in variable table. Check theme_&lt;THEME_NAME&gt;_settings under name column &amp; value is stored as serialized array in value column.
Update 1
To get file object use following sample:
$file = theme_get_setting('loader_img','tmhgs');
$file = file_load($file);