When i had my SE-C902, i discovered that I could hide certain pictures from being visible in the gallery or file manager. What I did was change the properties of the photo. (This is only a possible fix using windows, other OS's may work in a similar way)
select the photo you want to hide;
right click,
goto properties,
and you'll see 3 tick boxes, tick the box that says "hidden" then apply, then ok to exit. now the image itself is on the phone, but it should not be viewable in the gallery. I haven't tested this method yet on the N900 but it worked on my SE. let me know if this works for you.