[Fargo]

if you're not worried about the actual image file size, you could do the following:

Title: thumb1
Tag: thumb1
Replacement:

<a href="{param}" target="_blank"><img src="{param}" border="{option}" width="150"height="113"></a>

Example:
[thumb1=1]http://www.vbulletin.org/layout/nakkid.gif[/thumb1]

Description:

Takes a standard resolution image (image width is bigger than image height) and displays as a 150x113 px. thumbnail, linked to full size image.

Use "[thumb1=0]imageurl[/thumb1]" (no quotes) to show thumbnail without a border.

or

Use "[thumb1=1]imageurl[/thumb1]" (no quotes) to show thumbnail with a border.

NOTE: This is NOT a true thumbnail as it copies the original image and simply rescales it.  File size will be the same as the original.

Use Option: Yes