Well there are various parts to this, so if you aren't going to build everything, you need to decide what is important to you. At its core, there are several triggers, you could start out with just a single trigger. With only one trigger, you no longer need the LCD and the selection buttons to decide which trigger to use. For a lot of things you want to trigger the camera, which involves a specialized cable and some opto-couplers.
For water droplets/bullets/etc. the camera becomes the bottleneck, and instead you shoot with the camera in bulb mode (shutter open while the shutter button or cable release is held down), and you trigger the flashes instead. For really fast things like bullets, you have to get fast flashes and shoot at minimal power. It takes a lot of practice to get the timing just right.
Note, this assumes you are shooting with a DSLR that has a wired shutter release that can tell the camera to shoot via the wire instead of pressing the shutter button.
If you don't have such a camera, perhaps it takes an infrared shutter release, which you can simulate the shutter release with an infrared light. There is a library that provides the codes for most cameras that use IR sensors: http://sebastian.setz.name/arduino/my-libraries/multi-camera-ir-control/
. Note my experience on Olympus is there is always a noticeable delay when using using the IR shutter release as compared to the cable or pressing the button.
If you have a camera that has neither type of remote control, perhaps there is tethering support to fire it from a computer or wifi. It may be possible to fire such a camera from an Arduino, but likely you will need to reverse engineer the protocol.
Finally, if you have a camera with no such control, you would need to use a servo to press the button. Here is a video of one such device:
Another thing you need for high speed shots, is a camera that can be manually focused and shoot in manual mode. If you only have a camera that can shoot in automatic modes (i.e. a point & shoot camera), you will only be able to use the camera in non-high speed settings where there is enough light to focus on and assuming something interesting is in the point of focus.
Now assuming you have a camera that takes a shutter release cable, you need to get a cable that attaches to your camera. You can either get a pre-made shutter release cable complete with button for a few bucks from the usual Chinese merchants on ebay (with the usual long delivery time for cheap shipping). With this type of shutter release, you need to open up the wires, and separate the 3 wires. When two specific wires are connected, it does the 1/2 press action (typically focusing), and if you connect all 3 wires, it takes the picture (either for a set amount of time or in bulb mode, while the wires are connected). Alternatively, if you search around, you can find cables that plug into your camera, and have a standard 2.5mm or 3.5mm phono connection on the other side. Gadgetinfinity.com sells these cables, but you can find them elsewhere if you look. You will need a female plug that separates the 3 prongs into separate wires which you can find at an electronics store (while I've normally found them in more specialized stores, I have found them in Radio Shack, but you do have to look carefully in the cabinets in the back).
You hook up the Arduino to two opto-couplers, one for focus, one for fire. The opto-couplers are a form of protection to isolate your camera from the Uno and vice versa. Basically inside of the opto-coupler, is a light emitting diode (LED) and a photo sensor. So when the Arduino sets a pin high, it turns on the LED in the opto-coupler, which in turn turns on the photo-sensor, which completes the circuit.
Here is a picture of my telegraph key shutter release to give you an idea of the setup. In this, I don't have a trigger, but I connect a telegraph key. However, the telegraph key only provides on/off, and I wanted 3 states (off, focus, and fire), since my camera is somewhat slow at focusing when used in live view mode. So, when the telegraph key is connected, it enables the focus wire, and as soon as the key is released, it enables the fire circuit for a given period of time (which is controlled by a potentiometer so I can adjust the amount of time). As an added hack, I have the buzzer write out 'Fire' in morse code when the camera takes a picture. One thing that people have mentioned, and I need to re-do, is I need to add resistors on the connection to the opto-coupler, to prevent the opto-coupler LEDs from burning out. It isn't in the picture, but I have added a switch to control whether to do the buzzer action.
Here is a Fritzing image I made to show the logical connections:
Now, obviously you could replace the telegraph key with the trigger of your choice, and do both focus and fire at the same time.