Use Guides

The following quotes the article linked below (by Pat David):
https://www.gimp.org/tutorials/GIMP_Quickies/

Changing the Size (Dimensions) of an Image (Scale)

It's a common problem that you may have an image that is too large for a particular purpose (embedding in a webpage, posting somewhere online, or including in an email for instance). In this case you will often want to scale the image down to a smaller size more suitable for your use.

This is a very simple task to accomplish in GIMP easily.

When you first open your image in GIMP, chances are that the image will be zoomed so that the entire image fits in your canvas. The thing to notice for this example is that by default the window decoration at the top of GIMP will show you some information about the image.

Notice that the information at the top of the window shows the current pixel dimensions of the image (in this case, the pixel size is 1024x682).

This will then open the Scale Image dialog:

In the Scale Image dialog, you'll find a place to enter new values for Width and Height If you know one of the new dimensions you'd like for the image, fill in the appropriate one here.

You'll also notice a small chain just to the right of the Width and Height entry boxes. This icon shows that the Width and Height values are locked with respect to each other, meaning that changing one value will cause the other to change in order to keep the same aspect ratio (no strange compression or stretching in the image).

For example, if you knew that you wanted your image to have a new width of 600px, you can enter that value in the Width input, and the Height will automatically change to maintain the aspect ratio of the image:

As you can see, entering 600px for the width automatically changes the height to 400px.

Also notice I have shown a different option under Quality → Interpolation. The default value for this is Cubic, but to retain the best quality it would better to use Sinc (Lanczos3).

If you want to specify a new size using a different type of value (other than Pixel size), you can change the type by clicking on the "px" spinner:

Changing input value types.

A common use for this could be if you wanted to specify a new size as a percentage of the old one. In this case you could change to "percent", and then enter 50 in either field to scale the image in half.
Once you are done scaling the image, don't forget to export the changes you've made:

to export as a new filename, or:

to overwrite the original file (use caution).

Crop An Image

There are numerous reasons you may want to crop an image. You may want to remove useless borders or information for aesthetic reasons, or you may want the focus of the final image to be of some particular detail for instance.

In a nutshell, cropping is just an operation to trim the image down to a smaller region than what you started with:

Original image (left), cropped image (right).

The procedure to crop an image is straightforward. You can either get to the Crop Tool through the tools palette:

Or you can access the crop tool through the menus:

Once the tool is activated, you’ll notice that your mouse cursor on the canvas will change to indicate the Crop Tool is being used.

Now you can Left-Click anywhere on your image canvas, and drag the mouse to a new location to highlight an initial selection to crop. You don’t have to worry about being exact at this point, as you will be able to modify the final selection before actually cropping.

Initial pass with the Crop Tool. Crop Tool options (left), cropping on the canvas (right).

After making the initial selection of a region to crop, you'll find the selection still active. At this point hovering your mouse cursor over any of the four corners or sides of the selection will change the mouse cursor, and highlight that region.

This allows you to now fine-tune the selection for cropping. You can click and drag any side or corner to move that portion of the selection.

Once you are happy with the region to crop, you can just press the "Enter" key on your keyboard to commit the crop. If at any time you'd like to start over or decide not to crop at all, you can press the "Esc" key on your keyboard to back out of the operation.

Rotate and/or Flip an Image

There may be a time that you would need to rotate an image. For instance, you may have taken the image with your camera in a vertical orientation, and for some reason it wasn't detected by GIMP as needing to be rotated (GIMP will normally figure this out for you, but not always).

There may also be a time that you'd like to flip an image as well. These commands are grouped together under the same menu item:

If you want to flip your image, the Transform menu offers two options, Flip Horizontally, or Flip Vertically. This operation will mirror your image along the specified axis. For example, here are all of the flip operations shown in a single image:

In Conclusion

The simple examples shown here are just the tip of a really, really large iceberg. These are, however, common modifications that many people are often looking to make without having to learn too much about image processing. Hopefully they have been helpful.

I encourage you to peruse the other tutorials for more advanced methods of image processing as well!