The problem - images for the web - why I need cropping
I read on several web sites that the current optimal image size for blog posts is 1200 x 675. Whether this is true or not I'm currently following this advice. 1200 x 675 resolution has an aspect ratio of 16:9. The aspect ratio of 16:9 means an image will generally fit to cover the entire screen of most monitors when scaled to the monitor size. While I'm currently rocking dual monitors, both those monitors have a resolution of 1920 x 1080. This is also a 16:9 ratio. So if I take a screenshot of one of the monitors all I have to do to get it to 1200 x 675 is to use GIMP's Image > Scale Image > 1200 x 676.
My Canon Rebel XTi 10.1 MP camera is another story. Images captured in RAW mode on this camera are at a resolution of 3906 x 2602. If I try to resize the image I end up with 1200 x 799 pixels, not 16:9 and not 1200 x 675.
This is where the cropping tool comes in handy. My first step is to resize the image as described above.
After this step I used to just select the crop tool and try to watch the crop resolution at the bottom of GIMP until I got the 1200 x 675 resolution, or close to it. The problem is that it's difficult to get the mouse to move without going over or under those numbers. I did this for the longest time until just the other day when I came across a Davies Media Design video about another GIMP topic where one of the steps involved cropping the image. I noticed Michael (Davies Media Design) actually paid attention to the crop options, and that there are options to set the crop to be a "Fixed Aspect ratio" and size.
First click in the image (but don't apply a crop). Next click Fixed Aspect ratio, then enter the aspect size of 16:9, select a starting position of 0,0 and finally select a size of 1200 x 675. Now you can move the crop box around the image to get the best crop at exactly 1200 x 675.