From the Manual - Photoshop Elements

Straighten with the Straighten Tool

• Open an image (File > Open) with a horizon problem.
• Get the Straighten tool.
• In the Tool Options, click on the Remove Background icon.
• On the image, click and drag a line across the horizon, or where you perceive the horizon to be.
• Save the image (File > Save As) as a high quality JPG with a unique name.

Straighten with the Crop Tool

• Get the Crop tool.
• In the Tool Options, set the Ratio to No Restriction.
• Click on the third grid option.
• On the image, hover your cursor near a corner handle of the bounding box until you see that curve double headed arrow. Then click and drag until the image looks like it is lining up with the lines on the grid.
• Click the checkmark to commit.

From the Manual - Adobe Photoshop

• Open an image (File > Open) with a horizon problem.
• Get the Crop tool.
• In the Tool Options, click on the Straighten icon.
• On the image, click and drag a line across the horizon, or where you perceive the horizon to be.
• To adjust the tilt further, rotate the bounding box.
• Click the checkmark to commit. 

From the Transcript - Photoshop Elements

When traveling it’s not always convenient to stop and make sure your camera is level before snapping a photo. In this video, I’ll show you how to spot and straighten crooked photos. 

Images like this that contain a broad expanse of water like this stretches all the way to the horizon, are pretty easy to spot when they have a horizon problem. The horizon problem in this photo is very slight but it is still there. Let me show you how to fix it. This image is included in the downloads folder for this class.

Begin in Expert Mode of Photoshop Elements by getting the Straighten tool and then in the Tool Options click on the second icon, Remove Background. On the image click at the left side of the horizon and drag to the right side of the horizon. Then let go of your mouse. And just like that, Photoshop Elements has fixed the horizon problem. Here it was before and here it is now. 

Some images however are a little less straightforward. You sometimes will have a hard time deciding what actually in this image is the straight point. When you come across images like this, where you're not quite sure, the best thing to do is look smack-dab in the middle of the image. So what we want to make sure, is that these posts in the middle of the image are the thing that are straight. Again we could do that with the Straighten tool, but sometimes for images like this, it's better to use the Crop tool.

In the Tool Option set the Ratio to No Restriction and then we're going to turn on a grid overlay. You may need to move your program a little wider in order to see the options. I'll click on this third option that has the most boxes. If the grid doesn't appear over the image, click on the first crop selection to activate it. Then on the image, hover your cursor near a corner handle of the bounding box until you see that curve double headed arrow. Then click and drag until the image looks like it is lining up with the lines on the grid.

Now remember we're only looking at the center of the image. So that would be the polls in this building right here. And that looks like they are pretty lined up. Also it looks like this steeple is fairly lined up. But you might want to go a little farther this way. And that looks good to me, so I'll click the check mark to commit.

And now that this image is straight I would then go and fix the perspective problem. So that's how you straighten an image in Photoshop Elements.

From the Transcript - Adobe Photoshop

When traveling it's not always convenient to stop and make sure your camera is level before snapping a photo. In this video I'll show you how to spot and straighten crooked photos. Images like this that contain a broad expanse of water that stretches all the way to the horizon, are pretty easy to spot when they have a horizon problem. The horizon problem in this photo is very slight, but it is still there. Let me show you how to fix it. This image is included and the downloads folder for this class. 

Begin by getting the Crop tool. In the Tool Options click on the Straighten icon. Then on the image click and drag straight across the horizon. This will drag out a line that you should align with the part of the image that should be straight. So I think the edge of the water should be level. So I am going to go ahead and let go of my mouse when I get to the right side of the horizon and Photoshop will instantly straighten the image. And then it is going to ask me if I want to go ahead and crop it. I'll go ahead and click the check mark to commit. It was just that easy to go from crooked to straight. 

Some images however are a little bit trickier when determining what is straight and what is not. For example in this image the buildings over on the left side of the image appear as if they are relatively straight, but then if you go over here to the right side of the image, they are definitely tilted. I think this image is suffering not only from a straightness problem, but also a perspective problem. First let’s fix the straightness problem. 

If you start with straightness the first thing you want to pay attention to is the very center of the image, so it would be these buildings right here. We want to make sure that the posts in these buildings are straight up and down.

So again, I will click on the Straighten icon and click and drag on one of these posts as best as I can to try to make it straight. Now that looks quite a bit better, but you can notice that the trees over here on the left side of the image are really leaning in.

So I'll move my cursor over to this corner handle of the image and just tilt it slightly to the left here and the post in this image are pretty much still straight here in the center, but the trees don't look nearly as bad. And now that the straightness is problem is fixed, now you can go back to the first video and fix the perspective problem. So that is how to straighten photos in Adobe Photoshop.