Please trust me that your number one problem is that you are absolutely letting your hand and arms slip through the water, and that is the reason you have little propulsion per stroke. All other recommendations are secondary to this.

You have gotten a lot of advice.
Some good, and some crap. Unfortunately, a lot of the advice is in low priority areas.

Please trust me that your number one problem is that you are absolutely letting your hand and arms slip through the water, and that is the reason you have little propulsion per stroke. All other recommendations are secondary to this.

I recommend one drill to help you feel the water.
Swim one length with your fists closed so that you grab less water. Then open your hands to normal for one length and feel them grab (or slip through) the water. Keep doing this until you can feel them anchor the water and not slip through. Your underwater stroke will change over time as you learn to feel what grabbing the water feels like. You will also likely feel sore triceps or lats, as you train the muscles to actually work. Your stroke rate per minute will also likely come down until the muscles get retrained. Don't worry about that or over think it, just grab as much water as possible, no slipping. You can use a pull buoy to help isolate the pull if needed (but don't become an addict to the pull buoy).

After you get better at that, which could be immediately or weeks, then work on priority number 2, which is your stroke length. It is plenty long out front, but you wimp out at the finish and remove your hands to soon. You can push water another several inches down your hip and leg area. This will also force a bit more hip roll and hip power.

After you are great at this, go back and improve the water feel and grab, before moving on to some of the other stuff people have recommended, and you will be in a better position to judge the good from bad advice.

Others will probably pitch in with more expert feedback, but till then, here's my 2 cents.
Your body position looks good to my eye. Your biggest weakness is lack of propulsion. You should work on your catch and maybe your stroke rate as well. If you are into reading about this stuff, you might try the recent Sheila Taormina book that focuses on this aspect of swimming. You can look up some EVF drills; I like some of the sculling drills that you can find on youtube. Are you swimming with a master's group? If not, I'd recommend trying it. Good luck.