Need some Hardware advice with arduino
To process incoming images, voice commands, produce sounds, and also do WiFi is almost certainly beyond the capabilities of any micro computer, like the Arduino (maybe some of the more recent models might). I strongly doubt that the Mega 2560 would. It doesn't have a heap of RAM for one thing (it has 8k of RAM). Look at the sizes of images produced by cameras these days, they are usually measured in megabytes, not kilobytes.
What might work for you would be a Raspberry Pi, which is basically a small Linux computer on a board, with a substantial amount of RAM.