L298 output voltage is too low
Hmm. I actually selected this question from the suggestions on the sidebar. I did not realize its a year old!

Tie the enable pin to 5V for now - in this way the speed will just be constant. There is no need to try and fiddle with speed until you get the device working properly. 

It's hard to decipher what is going on based solely on the picture provided - I would consider drawing up the diagram and posting it instead. Also, post the code.

Now, considering the datasheet you can find this diagram:



As you can see, if you do not have the motor in place (between OUT1 and OUT2) then it makes sense that one of the pins is very close to 0V potential. Now, the other pin being at approximately the same voltage is an issue. 

Set IN1 to 5V, IN2 to 0V, ENA to 5V, and Pin1 (current sense) to ground (assuming you are using the left bridge) and see what happens (the upper left AND gate and the low right AND gate should be on). OUT1 should now have Vs (~12V in your case) when OUT1 and OUT2 are open circuit. 

Once you get this far you are on the right track. 

I am currently having a similar issue with my L298, so I cannot finish this answer but at least can provide guidance.