Thank you Acrylian,
This parameter is the way I used to limit the number of my images in my last images album (I've already posted something on the forum about a year ago about it and you greatly helped me

).
Now I will try to pass this parameter into a session variable.
I've another parameter to pass to get something different :
I'm adaptating my gallery to mobile device. In that way, I've made a differenciation between mobile and desktop use of my gallery :
- on desktop, each album shows on several pages of 12 images.
- on mobile, each album shows on one page, containing all images and the user (wich is used to scroll down on the tactile screen), have all the images accessibles by scrolling down.
Where my second parameter plays his game is when the user use the back button from the image page to go back to album. In mobile version, I need to send back my user to a certain portion of the album page instead of the top of the page. To make that, I've dynamically made anchors on each images (#ID1, #ID2, #ID3... #ID20...), and pass this parameter in the url. Then the link back head to : /portfolio/album/subalbum/#ID16 for exemple, and the viewport automatically come to the thumb of the picture the visitor just leave.
It's the way I find to do that, but it may exists something simpler (I'm very new in the coding world and maybe have wrong ideas, but, it does work...)
So if you have a better idea to do something like that, it's great. If no, no problem, I'll try to pass this parameter in a session too...
Thank you again for your help