Get thumbnails in custom fields?

<?php
$leeg = 'http://www.url.ext/images/noimage.png';
$var = get_nkm_imageurl('customfoto');
if ($var == '')
{
echo $leeg;
} else {
echo $var;
} ?>

I’m using the code above to display the .jpg / .gif etc. pictures that are uploaded via a custom_field. I’m using Thickbox to display these images in full resolution (fixed on the browsers resolution).

Now instead of linking to the original image that is uploaded, I want to link to the thumbnail that is created, for example:

– image.jpg (original image uploaded and linked)
– image-150×150.jpg (Thumbnail created by WordPress that I want to link to after the image is uploaded)

I actually have no idea at all on how to handle this… could anyone here give me some help? Or direct me in the good direction 🙂

The best solution would be to link to some ‘vars’ that give the following output:

– small_thumbnail
– medium_thumbnail
– big_thumbnail

I know these Thumbnails are standard in WordPress and can be configured as such.