Hello there,
I have a problem when I upload a big image in a K2 item.
The image size is 3.24 mb and the dimensions are: width: 6000px, height: 4500px.
Moments later, I see a blank page and the K2 item created without an image.
I would like to get an error msg dispalyed instead of a blank page.
Could you help me please.

Is there anybody else who experiences the same problem when he logs in as a registered user in the front end?

It most probably a memory limit issue.
In K2 's parameters there is an option to enforce a larger memory limit for image processing.

If your server setup is overriding these settings, you might need to upload a smaller image.

I agree with all you ve said, but the problem is not the image size. The problem is that when the user uploads a bigger image, we need a warning message to be dispalyed saying f.e. that the file is too big instead of a blank page.

This is a PHP memory error.
The script consumes more memory for processing a larger image than the limit set in your php.ini, it is not the filesize raising this issue.
This is how servers handle errors, so there is not much to do about this.

You can create custom 500 error pages and serve them instead of default white pages.
You need to consult your hosting provider if you have the ability to do this with your current setup.