How to force to upload a image with exact sizes? When adding an image field to a content type I can set the minimum sizes under which the image will not upload. I also can set maximum sizes but if the image is bigger the system will resize the image. How can I change this? 

I need to set both limits: minimum a maximun sizes. This mean a exact size for the image, then if the image has not the exact size it shouldn't be allow to upload.
This is mostly based on memory, so some details might be off.

The image field widget assumes that downsizing an image is a valid action in all cases, so you'll need to override it.

Implementing your own field widget requires a little bit of fiddling with the field api, but since your change is fairly small, you should be able to just wrap a number of image field function and add a single validation step.

Since image field already is mostly a wrapper around a file field, looking at the image field itself for how to implement the above is probably a good start.