May be some explanations are in order:
1) I'm working on my own theme, that works for all the photographies (about 90% of the gallery)
2) for the remaining pictures files, such as postcards, press-book, graphic work and such, I use the multiple-layouts plugin with a custom album page, and a custom image page
3) I'd rather avoid different custom pages, because of future maintenances; things are too similar from one type of pictures to the other, so I'd rather find some solution using either custom data or code block on specific albums
4) entering a list of numeric values in the custom data doesn't seem very user-friendly / reliable, that's why I'd like to link each value to either a meaningful label (width, height, and such) or use the code block to assign a value to each variable.
@acrylian: I tried to use global variables, but it didn't work. I probably dit it wrong... can I find somewhere an example of codeblock in that manner?
And by the way, I didn't find the way to switch the language in the admin interface: when I choose English in the general options tab, the interface remains in French!