The first step should be deciding on the purpose of the edited image - is it for Print or for Web Use?
A printed image should be 300 pixels per inch and, of course, you may be restricted to certain sizes.
Since an image to be displayed on a monitor or tablet or mobile phone does not require the same resolution the image should be 72 pixels per inch. The image can be any size you want but, of course, work within the contraints of monitors etc.
I will be concentrating this blog on images for web use. Although I will probably update at a later stage to include images for Print.
Open your image in Photoshop
As you can see there is a lack of colour in the image so lets adjust the "Levels" so click on "Image" > "Adjustments" > "Levels"
Play around with the three sliders underneath the graph until you get the required result.
Right let's look at the size of the image. You do this by clicking on "Image" > "Image Size"
This is one big image 4752 pixels x 3168 pixels 72 pixels to the inch and comes in a a whopping 43.1 megabytes.
Now it all depends on the size of the image you require for your website - lets say 1500 pixels x 500 pixels.
Click on your Cropping Tool
and adjust the width to 1500 px height to 500 px and resolution to 72. Move your cursor to anywhere on the image and drag until you are satisfied with the crop. Of course it can be adjusted until you are happy with the result. Then apply the crop.
Lets look at the image size now.
Ummmh! still a big file 2.15 megabytes. Too much for our website. The final stage is to save the image but we want to cut down the size of the image so click on "File" > "Save for Web and Devices"
I have gone for JPEG preset High so there is a little bit of loss of quality but look at the size of the file 129.4K as opposed to the original image of 43.1 megabytes or 43,100K so the image now as a fraction of the original is 129/43100 or 0.3% of the original. Now granted the image is a lot smaller but I think you get the picture.