You haven't provided much info. First white thing is labelled as transformer for "LV Halogen Lamps". Most likely it's just a step down transformer which will fry your christmas LED strip during reverse polarity.
Second one is a mystery box without much info. I wouldn't go for that either.
Here is what you need to do:
Figure out what voltage is required by your LED strip - 3V, 5V, 12V or something else?
If the LED strip is 5 watts, then you can calculate the current requirement as follows:
Current = Power / Voltage required.
Ex- If LED strip needs 5V, then you will need 5W/5V = 1A of current.
Go to the market and ask for a 5V 1A(or higher) wall adapter. Modify the voltage and current accordingly. I am just giving an example.
Get something less intimidating like this:
EDIT after the question was edited:
Figure out what length of strip you want to use. I will assume 0.7 meters which will give you total power requirement:
Power = Length X Power required per unit length = 0.7 X 7.2 = 5 watts.
Voltage = 12 V
Current = Power / Voltage = 5/12 = 0.42 A
Better to go for 12V 0.5A (or higher) wall adapter or any DC power source. Make sure it's a DC power source and not just a step down transformer. If the power source is DC 12V 12W, then it will do the job without any issues.