I saw so many images on internet with Tear on eye and sad faces. So i tried to make a tear by using photoshop, lets start the tutorial. Our final image can be like this...

First of take any image which you want to use in your project. I am using this one

Create a new layer, we will make Tear on it.

Now use brush tool and put a small dot below the eye as shown below.

Set the dot layer style to the Screen as shown below

Now go to the blending options of the layer and apply these settings.

Drop Shadow

Inner Shadow

Bevel and Emboss

Now your Tear should look like this
Increase the Tear size and decrease the opacity to 50%

Now your Tear is ready, now go to Filter > Liquify. Use push tool and drag the eyebrows to little bit up to make the face sad. (because a baby cannot cry and smile at the same time :P)

Now your image is ready, you can change Tear position to make it more suitable.