How to require Minimum Image Dimensions?

Hi there,

I recently bought a theme which has some kind of a gag like feel.
Users can post their own images and videos to share with others.

I have a problem however, and that is that i’m not able to require a minimum image size (dimensions), and since the theme automatically stretches uploaded images, this make small images very blurry.

That’s why i want to restrict my users from uploading small (below 400×300 for example) images, to prevent image tearing.

I’ve read a post which suggests to add a few lines of code to the themes functions.php file, but it does not seem to work for my theme/wp version.

It’s this post:
http://wordpress.stackexchange.com/questions/28359/how-to-require-a-minimum-image-dimension-for-uploading

The only thing that works in this solution is that it prevent’s the image from uploading, however the post is still published but just without the image.

What i need is the user from receiving a message that the image dimensions are too small and i want them to be able to click a link which goes back to the uploading form.

Is there somebody that has experience with this issue?

Thanks in advance.

Danny