Change Default Thumbnail Size in WordPress

I’ve been using WordPress for a few weeks now, and thinks it’s an awesome blogging tool, however, as with most applications it doesn’t do exactly what I want it to do. One of the annoyances I have right now, is that the thumbnail image WordPress 2.3 creates is too small for my tastes and so I wanted to find a way to change this. It is actually quite easy to change – it just requires modification to one WordPress file.

Open /wp-admin/includes/image.php and look for the following code:

$max_side = apply_filters( 'wp_thumbnail_max_side_length', 128, $attachment_id, $file );

The number specified here (128 by default) is the maximum size of either dimension. Just change this to whatever you like. I’ve changed this to 256 and works really well (below)