I use a truckers hitch
it is easy to make and create and pull tight.
It is not difficult to untie but does stay in place well.
It is a great knot when you need to cinch something down.
Image source https://commons.wikimedia.org/wiki/File:TruckersHitchUsingAlpineButterfly2.jpg
outdoors.stackexchange.com/users/4079/James Jenkins
I agree that the trucker's hitch will certainly do the job.
That said, if your special situation requires retightening if things start to sag, you might consider the tautline hitch .
It's a great knot for situations where you might need to take up slack due to stuff like rope stretch in the dark and rain (like, say, if you're using your line to make an A-frame shelter with a tarp), so you can get back under cover quick.
I wouldn't recommend this knot for situations where a knot slip might cause injury.
As always, right knot for the right job.
outdoors.stackexchange.com/users/18497/Scott Seidman
As others have commented, Trucker's Hitch (with an Alpine butterfly) would be the best.
Note that it would be enough to do it on one side only; I usually tie an Anchor Hitch at the other side.
Another option, when you won't be loading the rope too much, would be to use an Adjustable Midshipman's Hitch .
As a bonus, you could easily re-tighten the rope by pulling the adjustable knot.
I tried this last time to dry the clothes between the trees (just like in your picture) and it worked very nicely:
the rope was taut but when the wet clothes were put on it it sagged a little.
Then I just pulled the knot - et voilà !
outdoors.stackexchange.com/users/14952/Alexander
You don't need a knot, just add a few rounds of rope around the tree and the friction will make it more or less impossible to "pull" the rope and create sag (besides, a few extra rounds prevents the rope from sliding downwards).
Basically any knot will work after this although two half-hitches is a good option.
An alternative to the extra rounds is to simply make a clove hitch around the tree.
That can be done even under stress.
outdoors.stackexchange.com/users/15680/d-b
If you're after getting tension into the line, I'd normally use a Truckers Hitch .
This uses a loop to create a pulley - and there are various ways to make the loop.
I tend to prefer either an Alpine Butterfly (as it doesn't get hard to undo after tensioning) or the loop of a slippery overhand knot (as it can be undone easily).
If you have issues losing tension when tying off the truckers hitch, you can opt to use a "wrap and re-wrap" method.
Not sure that's the right name.
Basically you pull the rope round the tree, round the taut line and back round the tree (back the way it came).
You can repeat this, going back and forth round the tree, any number of times.
Then you tie off the loose end to the taut line.
This wrap and re-wrap method can be seen demonstrated about 90 seconds into this video: https://youtu.be/zdn1MlOUFuA?t=90 - albeit without doing a truckers hitch first.
outdoors.stackexchange.com/users/18063/Grhm
Say, I want to tie a rope between two trees, and I want it to have as little sag as possible. I have attached the rope to one tree and I'm trying to tie it to the other tree  while pulling it to create tension and remove sag .  All the hitch knots that I know are difficult to do while applying tension and, when tightened, create some sag.  Which ones are easy to do under tension and avoid sag?  Thank you.
Most practical knots for hitching a line to an object while keeping the bitter end as tight as possible, without sag?
outdoors.stackexchange.com
<knots><ropes>
outdoors.stackexchange.com/questions/22911
outdoors.stackexchange.com/users/18485/Andrey Mikhaylov - lolmaus
A trucker's hitch is widely recommended, although a clove hitch, an alpine butterfly, a slippery overhand knot or an adjustable midshipman's hitch could be used. Another approach would be to add a few rounds of rope to hold by friction followed by almost any knot. Alpine butterfly,  Adjustable Midshipman's Hitch or clove hitch around a tree work well. Any knot or a few rounds of rope around a tree can work. A trucker's hitch works best.
A trucker's hitch is best. Alpine butterfly,  Adjustable Midshipman's Hitch or clove hitch around a tree work well. Any knot will work, or putting a few rounds of rope around a tree.