In functions.php you can change the thumbnail size - just open the file and search for following entries:
#####################################################################
# Define Thumbnail sizes
#####################################################################
$k_option['custom']['imgSize']['base'] = array('width'=>267, 'height'=>180); // backend preview size, if changed does not affect the frontend
$k_option['custom']['imgSize']['S'] = array('width'=>70, 'height'=>50); // small preview pics eg sidebar news
$k_option['custom']['imgSize']['M'] = array('width'=>192, 'height'=>130); // medium preview pic for portfolio with 4 columns
$k_option['custom']['imgSize']['M3'] = array('width'=>274, 'height'=>170); // medium preview pic for portfolio with 3 columns
$k_option['custom']['imgSize']['M2'] = array('width'=>436, 'height'=>230); // medium preview pic for portfolio with 2 columns
$k_option['custom']['imgSize']['L'] = array('width'=>574, 'height'=>268); // image for blog posts and 1 column portfolio
$k_option['custom']['imgSize']['XL'] = array('width'=>940, 'height'=>440); // big images for fullsize pages and mainpage slider
Change the dimensions to your requirements.