Mantis: What can I change to automatically display attached images when viewing issues?

The behavior is controlled by a variable $g_preview_attachments_inline_max_size

in config_inc.php

According to the Mantis Guide :

This function applies to bmp, jpg, gif, and png files attached to errors. If the file size is less than the specified values, the actual image is also viewed with errors [...]

Although the manual states that the default is 0 (feature disabled), in Mantis 1.2.0 it is 256KB:

$g_preview_attachments_inline_max_size = 256 * 1024;