Each achievement should have two images: a small image and a large image. Both of these involve taking screenshots from the achievement viewer.
Suppose game "Cool Game" has achievement "Do This". The small image should be titled Do_This.png which is 38x38 pixels and is equivalent to taking a screenshot of the achievement (with the yellow border), in the bottom part of the official achievement viewer. This image does not need transparency as it is on a blue background with a yellow border. There should also be a large image Do_This-large.png which is 128x128 pixels and is equivalent to saving the large image from the top-right of the official achievement viewer and converting it from .gif to .png. Both images should use the file ending ".png" and not ".PNG". The large image should then be linked for use in the respective infobox.
The small image will appear in the game's achievement table automatically.