0 x 0 pixels: html doesn't apply their proper dimensions

I don't know why, but the browser thinks that the images must be "shown" with 0 x 0 pixels resolution. Nevertheless the browser actually does know that, let's say "promotoras-1.jpg", has a resolution of 800 x 600.

I saw this question (Image in <img> tag is defaulting to 0 x 0 pixels despite setting its size) but that's not exactly my case (I'm not using a HTML5 video tag here).

Is there a way to fix that?

EDIT:

I didn't set any height or width for the images. But I'm using both, Lightbox and Nivo-Slider together. It worked for me before, but then this happened. I've tried deactivating the scripts, and also played with the inspector with the properties, deactivating all of them, just to see if the images would still show up... but it's still the same result.

EDIT 2: Link to the site

Yes, I've found the mistake: I didn't call the function nivoSlider();.

Also make sure that the jQuery version you're running isn't below 1.9.1