What if I dont use a USB cable but rather use wi-fi ?

Short answer is: you can't move photos through Wifi directly that's why there are no instructions.

Long answer: There are certain Apps in the App store that will allow you to send Photos over Wifi to another device or PC.

Search for "Wifi Photo Transfer" in the App store for some options.

just use dropbox.

It sounds like you need to setup Wi-Fi syncing. Have a look at this article and expand the section under

Wi-Fi syncing

http://support.apple.com/kb/HT1296

Wifi Syncing will not transfer pictures from iPhone to the computer.

Dropbox is a Cloud based service. Its basically a service that gives you a specified amount of storage space in their servers, and you can upload files to it, and then download from it.

There is an App in the App Store for it.

What you do is upload the files to a server on the internet, which you can then access form your PC.

Wifi Photo Transfer Apps let you skip the upload and subsequent download steps to and from the cloud server by transferring directly to the computer.

Again search for Wifi Photo Transfer.

After you read and enable the instructions in this post, no 3rd party app will be necessary. Go to settings>iCloud>Photo Stream. Set Photo Stream to "On." As long as your Mac is logged to the same iCloud account you'll have access to all of your future images within minutes after they've been snapped.

There is a way to get existing images to photo stream as well. With Photo Stream enabled, create a shared album. The pictures you place inside that album will show up in the Photo Stream.