Oh yes!
Thank you so much, that was it!
I've had to spend yesterday playing around with the mosaic dataset possibilities and using some polygon from another shapefile (containing the name of the picture as an attribute) as footprint and got some really good results but it is much more work!
Not sure which way I go for the rest of my work:
-I think when picture don't overlap apart from the white border the Mosaic tool within the Image Analysis window (using minimum for the output) is plenty good enough.
-When picture overlap including area of data a mosaic dataset is needed.
Thanks again for you're help, after hours I did give up looking for it!