Your photoshop crop tool is not only to cut images, you can create precisely sized thumbnails. And this is what I am going to show you.
You use the crop tool by clicking on it with the left button of the mouse while at the same time dragging it to form the desired rectangle and releasing the button.
... in action (the dotted yellow highlighted line is for educational purpose and does not show as such in Photoshop where you can see a simple dotted line)
Of course the idea how to crop follows the rules of composition. And so for instance, keep the horizon at 1/3 from the top or bottom of the picture, have a person's eye in one of focal points (see below) etc...
The rule of thirds is way to mentally (and physically on many camera viewfinders) see a proper, simple grid diving the image in front of you in a simple grid with 1/3 fractions...Usually, the focal point is at (or close to) the intersections.
Without the grid...
..and with the grid
Same story with a different grid. If you want to know more about the Golden section, click here.
The Golden section gridDownload the Rule of Thirds Grid (ZIP file) to be used in your photo-editing programm
Download the Golden Section Grid (ZIP file) to be used in your photo-editing programm
In photoshop I open a picture, then I "File>place..." the grid over it and adjust it to the picture ("edit>free transform...", or CONTROL + T)
Thumbnail option: set the desired cropping size in whatever masure you need, cm (as in picture), pixel (write "px" like "60px").
Alternatively, you can crop an image for desktop backgrounds sizes:
All numbers are in Pixels
|Standard Screen Sizes||
800 x 600
1024 x 768
1400 x 1050
1600 x 1200
1920 x 1440
2048 x 1536
|iPhone Screen Sizes||
320 x 480
480 x 272
|iPod Touch Screen Size||480 x 320|
|iPad Screen Size||1024 x 768|
|HD Screen Sizes||
1680 x 1050
1920 x 1080
1920 x 1200
2560 x 1600