Hi bigxero,
you also might want to use the add-in phpThumbOf in your template. phpThumbOf scales your image into the properties you want; thus assuring your postThumb-image will not mess up your layout with for instance a way to large image.
make your postThumb TV > input: image; output: text
<div class="post_thumb">
<a href="[[~[[+id]]]]">
<img src="[[+tv.postThumb:phpthumbof=`w=602&h=224&zc=1&q=80`]]" class="pic" />
</a>
</div>
The above will output your blog-image as zoom-cropped, quality 80%, width 600px, height 224px.
Use this code in your blog (overview) page. For your single-page blog-item change the above code: remove `+tv.` and replace with `*`
More info about phpThumbOf ->
http://rtfm.modx.com/display/ADDON/phpThumbOf
Happy blogging!