You ask about "cropping". Cropping is the cutting-off of one or more sides of a photograph, either when printing it, or preparing to email it. That is, you crop-out or crop-off (or cut-out or cut-off) the less interesting parts to highlight what is interesting, so as the viewer's attention is not distracted by unimportant detail and the eye is drawn to what is important in the original photo. This is also achieved by "blowing up" a section of the photo's negative.
Good cropping is one of the quiet skills of the trade. Most photographs carry dead space around the edges, a stretch of empty sky, a cluttered background, a parked car that means nothing. Trim it away and the subject suddenly stands up and speaks. The art is to ask, of every picture, what is the one thing this photo is about, and then cut until that thing fills the frame. A tight, well-judged crop can rescue an ordinary snap and make it look professional.
A few practical habits help. Crop to lead the reader's eye to the face or the action, not to the lamppost behind it. Keep the shape your page needs, since some slots are tall and some are wide, and crop with that in mind. And never enlarge a small section so far that it turns to mush; a sharp small picture beats a big blurry one every time.
Now the warning, the same one that hangs over every photograph. Cropping must never change the truth of the picture. It is one thing to trim a dull background; it is quite another to crop a person out of a scene so the meaning flips, or to cut away the context so a peaceful gathering looks like a brawl, or the other way round. The classic sin is cropping out the one detail that would change how the reader understands the whole image. Trim for clarity, never to deceive. The reader trusts that what is left in the frame is a fair account of what the camera saw, and that trust is yours to keep.