Thumbnails problem GK NEWS

Hello,

I have a problem with the thumbnails in the GK NEWS module on the web site www.snap-and-roll.com. Depending on the image inserted in the article sometimes i get these errors(See below). If I change the picture and I am lucky sometimes the thumbnail is created, but of course I would like to display the thumbnails with the picture I want. I tried to resize the picture and even lower the resolution, but still do not work.
If i go to Image Setting in the GK NEWS MODULE and I disable the thumbnails option it works perfectly with any picture but then the EDIT BAR (allows you to customize number of articles shown) turns DISABLE.
Any idea how to figure it out?
Thank you in advanced.

Warning: imagesx(): supplied argument is not a valid Image resource in /home/www/snap-and-roll.com/modules/mod_gk_news/helpers/image.helper.php on line 177

Warning: imagesy(): supplied argument is not a valid Image resource in /home/www/snap-and-roll.com/modules/mod_gk_news/helpers/image.helper.php on line 178

Warning: imagecopyresampled(): supplied argument is not a valid Image resource in /home/www/snap-and-roll.com/modules/mod_gk_news/helpers/image.helper.php on line 221

Warning: imagepng() [function.imagepng]: Unable to open '/home/www/snap-and-roll.com/modules/mod_gk_news/cache/' for writing: Is a directory in /home/www/snap-and-roll.com/modules/mod_gk_news/helpers/image.helper.php on line 225