Here is the solution :
Edit the following file, using Internalz :
at the begining of the file, you'll see 2 var named ScreenNail_WIDTH and ScreenNail_HEIGHT : change them to whatever resolution you want
Save, close, reload Luna
Remove all the screenNail pictures in your /media/internal/.photosApp/Generated
Don't remove the thumbnails files.
If you do, you'll have to go through all your albums to force a regenerate of every thumbnails. Album thumbnails will show OK after you regenerate every thumbnail in the album itself.
hope that helped