gallery (shortcode) images size problems

Another day another problem…
This time, my the images in a gallery have all the same size. If I set image size 280px x 280px all images have this size. css says:

img {
height: auto;
max-width: 100%;
}

But: Some images have their max-height, some their max-width but they are all 280px x 280px.
If I change it to 280px x 600px, its the same – but different ??

Am I missing something? I tested once the gallery shortcode and it worked. I didn’t change anything sice then. I deleted the costum css codes just to be sure…Can somebody help? Why is there a setting for height anyway if the height is set “automatic”? It seems to me like there is a conflict between the setting of the shortcode and some other “code” (sorry, but for me the custom-css plugin-thing is already “complicated”).
I would realy appreciate your help!!
thanks