I found this "problem" when the first image uploaded is very small.

When the images are shown grouped in the activity stream and the first one (I have not tried uploading only small ones to see what happens) the first one is shown in the center of a grey background, when the image is big enough it shows good filling all the space. Check this image, I tried this in your demo site.




I think that can be fixed making automatic zoom for smaller images filling the background space. Like this:




Or placing the image with a background of the same size and sticked to the botton above the other images. Like this:





I tried to modify the css using firebug but couldnt find a solution for this displaying "error". What do you think about this? Thanks!

That's not a problem. It's designed that way. CB Gallery does not upscale images. It will only downscale if the image can not fit in the space provided. In the case of Activity if you upload multiple photos at once it groups their activity with the first entry being larger than the others. You're welcome to restyle it however you like using CSS or overriding that activity template file for CB Gallery. There are no plans to change this design at this time.