What an image looks like depends on two things: the scale -- the relationship of the size on the photo to the area covered on the ground, and resolution -- the detail and clarity of the image. Remote sensing means observing or sensing things without touching them. Our eyes do this all the time. But what something looks like depends on how big it is and how close you are to it. These two things will determine the scale.
For example, if you hold a book about arms length away you can read the words on the page easily. If you touch the book to your nose, you might be able to identify a few letters, but mostly you just see blurred black blobs of print. And if someone were to hold the book for you across the room, you could probably see it is a book that has writing in it -- but that would be about it. Images are the same way. Some images are taken up close and others from far away.
If you've ever used a camera you know what this is like. Let's say you were told to get a picture of flowers. Close up a picture will give you a big image of a small flower, and it will take up your whole photograph. A picture taken from far away would give you the same size photograph, but the flowers in the picture would look tiny, and you will probably get a whole hillside covered with them!
Sensors are usually place up above what they are looking at. Detection instruments are placed on platforms to hold them still and steady. These platforms holding the sensors may be hand held or on a truck on the ground, in the belly of an airplane, or in a satellite in space. An image taken from the ground might show an image of a farmer's field in the center of the image. From an airplane, an image with the same field in the center would show not only the farmer's field, but other fields, farms and towns nearby. It would show the whole area. From a satellite, an image with the farmer's field in the center would likely show the whole state!
As the area in the image covers more and more area on the ground, we say the scale is getting smaller because the things in the picture are small. A large scale image has something large in it, but it would cover only a small area on the ground. The picture of a single flower from the example above is large scale, but covers only about 1 inch on the ground. The picture of the hillside of flowers is small scale, because it covers a large ground area but the objects in the picture, the flowers, are small. Scale is used used in maps, in photographs and in remote sensing.
In remote sensing, most of the time the objects are being looked down on from above. Things look different when viewed from on top. Our eyes see things straight ahead, from the side. Only when we look directly down on things from above do we see what they look like from on top. Everyone knows what a water glass looks like. It is often clear glass or plastic, sometimes with a design on it. It is a round cylinder, hollow in the center to hold things. Look at it from on top and just looks like a circle!
Imagine how odd things must look from space. When an astronaut (or a remote sensing instrument) looks down at the Earth it sees the tops of things. Houses look like square and rectangles. Trees look like circles. Streets and bridges and rivers look like curvy lines. Whole mountain ranges just look like wrinkles! And the Earth itself looks like a big, blue marble.