Try with width:auto !important;

It worked, but did not solve my problem problem completely.

The image took over the default size but still will not let me change the size.

More precisely, if I change the image size using the category default sizes large, medium, small, and so on, the image takes on the dimensions that are defined in the default parameters... but if I change the dimensions in categories parameters, the new dimensions are not recognized keeping the originals.

Another thing, when I changed to "width: auto !important" the image was centered, but applying "float: left" was fixed.

PS: I also tried using the "inherit !important" but the result was the same.

OK. The image once uploaded is being resized into 5 sizes according to the K2 Parameter sizes. Which means that if you are loading the Medium size and the image has size 400px when you put a different size through the category parameters the image size doesn't change. In fact the new width size is just added through a style="width: category_image_width" in your case 100px;
In order for this size to override all others you need to edit the category_item.php file of your template overrides and locate this codeand change it to this code
This will tell your image to always be sized according to the size you provide through the category parameters.

This correction in addition to having solved the problem also allowed me to dispense override it had done previously.

I also noticed that this problem with the image default sizes also happened with the item ... I did the same correction in the item.php file and also solved the problem.

thank you! :)

You are welcome.