I think it is also worth mentioning that the rituals we are talking about are rather complicated high-level magic, you don't just take a photo of somebody sit in a circle and stare daggers at the picture. It's not like any Tom, Dick, and Harry turned Warlock can just go and whip them out.
They usually require White Council level of skill, power, and training. Which in itself would minimise the pool of suspect would you investigate a thaumaturgy related murder. Not to mention that there are always ways to reduce the suspect pool further. Black Magic leaves traces you can find, physical distance is also a factor in this spells, so it's not like you could literally kill someone who is Hawai from Moscow, the energy expenditure would be too great. You have to believe you have a right to kill that person so you most likely know them and hold a personal grudge against them that even a vanilla investigation might find out.
The only person we have seen use that kind of thaumaturgy outside the WC was Victor Sells, and he was as it later turned out sponsored and coached by the Red Court or someone from the Circle. It's not something the average warlock will get his hands on, and even if, it is even lees likely they will have the juice to pull it off. Sells had to wait for thunderstorms to roll in, while having fresh blood, skin, and hair of his victims, the perfect thaumaturgical channel, despite having the closest a Warlock can hope for in actual magical training.