When I upload an images to an article the K2 resize the image. My original is 830 px and K2 resize it to 400 px. The funny thing is that K2 shows the image in the top as a resized image from 400 px to 830 px. As if it was an original at 400 px.
Where can I control the size? I found the configuration an images tab but nothing happens when I try to ajust the figures.

You can choose which image sizes is used in the category's settings.
The sizes are defined in K2's params.