How to resize a picture to 1200 pixels wide
FTP by Zaweba's Resize Image has a 1200 wide preset with Keep proportions, so a big photo becomes the right size for your website in one click.
In FTP by Zaweba, open the picture, choose Resize Image, make sure Keep proportions is ticked and click the 1200 wide preset. The width becomes 1200 pixels and the height follows on its own, so nothing is stretched. Then save, or better, use Save for Web.
Twelve hundred pixels is a sensible width for a lot of web pictures: wide enough to fill a main column on a laptop, small enough that it doesn't take an age to load on a phone. Photos straight off a camera or phone are usually several times wider than that.
The quick way
Images open from the file trees or with File > Open into the image editor built into FTP by Zaweba. Open your photo and choose Resize Image. You will see the width and height, a Keep proportions option and a row of quick presets: 25%, 50% and 75%, and 800, 1200 and 1920 wide.
Tick Keep proportions if it isn't ticked, then press 1200. Apply it and look at the picture. Zoom has fit and actual pixels (1:1), so switch to 1:1 to see how sharp it is at its real size. If something is wrong, Undo takes you back.
Choosing between the presets
The 1200 preset suits a main picture in a page's content area. Use 800 for pictures that sit beside text or inside cards, where a bigger file is wasted. Use 1920 for full width banners that span a large screen. The percentage presets are handy when you want something smaller without caring about an exact number, such as halving a screenshot.
You can also type your own width. With Keep proportions on, the height is worked out for you; with it off, you set both, and the picture will be squashed or stretched unless the two numbers keep the original shape.
Crop first, then resize
If the photo needs a different shape, crop before you resize. Drag a box over the part you want, choose its shape and press Enter to apply. Rotate 90 degrees either way or flip it if it came out on its side. Resizing last means the 1200 pixels you end up with are all picture you want, not edges you were about to throw away.
Saving a picture that is small in bytes as well as pixels
A 1200 pixel photo can still be a heavy file if it is saved at the highest quality. Save for Web (File > Save for Web, or Ctrl+Alt+Shift+S in the image editor) is made for this step. Pick the format and quality, with 60 to 80 usually indistinguishable on screen, and watch the file size in the live preview as you change them. It also has an optional maximum width, so if you prefer you can skip Resize Image and set 1200 there: the picture is scaled down to that width with its proportions kept.
Save for Web removes camera details and location from the copy, which is a good thing to do to any photo going on a public site.
Putting it on the website
Save the resized copy into your site's local folder, ideally in its images folder, and upload it with Put, or drag it across to the server pane. If the page already uses an image with the same name, the new one replaces it, and FTP by Zaweba keeps the previous version in its local history before a Put overwrites the server copy.
Then open the page in Live Preview (Ctrl+Shift+P) and try the device sizes, phone (375) up to desktop (1920), to see how your 1200 pixel picture behaves in each. If it looks soft on the widest size, the 1920 preset was the better choice; if it is only ever shown small, 800 will load faster.
Doing it with another tool
Any photo editor can resize to a width, and Windows has basic tools too. Doing it in FTP by Zaweba saves a round trip: the picture is resized, compressed and stripped of its location in the same app you use to upload it and check the page.
