I've a suggestion, which I'm just going to throw out there.
Rather than weighting the scores in a preset manner, I would suggest a system whereby the theme score defines the limit on the other scores. For example, if I take a technically and brilliantly impacting photograph which has no relevance to the theme, I may score 11 on the theme. Under my system, the tech and impact score could be no higher than 11. Assuming maximum marks in the tech and impact department, I would get 33.
This would then prevent people scoring low in the theme but very highly in the tech and impact. It would force people to think about improving their theme mark, first and foremost, with tech and impact being considered after the theme relevance is considered.
Any thoughts?