What's a pixel?

Numbers.   A digital color image pixel is just a RGB data value (Red, Green, Blue). Each pixel's color sample has three numerical RGB components (Red, Green, Blue) to represent the color. These three RGB components are three 8-bit numbers for each pixel. Three 8-bit bytes (one byte for each of RGB) is called 24 bit color. Each 8 bit RGB component can have 256 possible values, ranging from 0 to 255. For example, three values like (250, 165, 0), meaning (Red=250, Green=165, Blue=0) to denote one Orange pixel. Photo editor programs have an EyeDropper tool to show the 3 RGB color components for any image pixel.

In the base 2 binary system, an 8 bit byte can contain one of 256 numeric values ranging from 0 to 255, because 2 to the 8th power is 256, as seen in the sequence 2,4,8,16,32,64,128,256. The 8th of these is 256. This is the same concept in base 10, that 3 decimal digits can store one of 1000 values, 0 to 999.   10 to the 3rd power is 1000, same idea as 2 to the 8th power is 256.

Yeah, right, but the only point here is that 255 is the maximum possible number that can be stored in an 8 bit byte. Larger numbers require multiple bytes, for example two bytes (16 bits) can hold up to 256x256 = 65536 unique values. 24 bit RGB color images use 3 bytes, and can have 256 shades of red, and 256 shades of green, and 256 shades of blue. This is 256x256x256 = 16.7 million possible combinations or colors for 24 bit RGB color images. The pixel's RGB data value shows "how much" Red, and Green, and Blue, and the three colors and intensity levels will be combined at that image pixel, at that pixel location.

The composite of the three RGB values creates the final color for that one pixel area. In the RGB system, we know Red and Green make Yellow. So, (255, 255, 0) means Red and Green, each fully saturated (255 is as bright as 8 bits can be), with no Blue (zero), with the resulting color being Yellow.

Black is a RGB value of (0, 0, 0) and White is (255, 255, 255). Gray is interesting too, because it has the property of having equal RGB values. So (220, 220, 220) is a light gray (near white), and (40,40,40) is a dark gray (near black). Gray has no unbalanced color cast.

Since gray has equal values in RGB, Black & White gray scale images only use one byte of 8 bit data per pixel instead of three. The byte still holds values 0 to 255, to represent 256 shades of gray.

Line art pixels are represented by only one binary bit with values 0 or 1, used to denote Black or White (2 colors, no gray). Line art data is stored packed 8 bits into one 8-bit byte.

What's in an image file?

Those numbers.   The image file contains three color values for every RGB pixel, or location, in the image grid of rows and columns. The data is also organized in the file in rows and columns. File formats vary, but the beginning of the file contains numbers specifying the number of rows and columns (which is the image size, like 800x600 pixels) and this is followed by huge strings of data representing the RGB color of every pixel. The viewing software then knows how many rows and columns, and therefore how to separate and arrange the following RGB pixel values accordingly into rows and columns.

Every location on one of the rows and one of the columns is color sample, which is called a pixel. If the image size were say 1000x750 pixels (written as width x height by convention), then there would be 1000 columns and 750 rows of data values, or 1000x750 = 750,000 pixels total. For 24 bit color, each pixel's data contains three 8-bit RGB byte values, or 750,000 x 3 = 2,250,000 bytes. Every pixel is the same size, because a pixel is simply the color of the area between the grid lines. That area will be colored by the one corresponding RGB data value. Larger areas of the same color are just many multiple identical pixels, including the blank background (the blue sky at right), which are many more pixels too. The image data is just a series of RGB numeric color values in a grid of rows and columns.

The image itself is an abstract thing. When we display that color data on the screen, then our human brain makes an image out of it from the appearance of all of these RGB data values.

Icons are usually "graphic" images, built of discrete pixels, instead of having continuous tones like photographs. Some graphic artist has worked very carefully on the previous icon, pixel by pixel, one pixel at a time. But a photograph is more blended, and adjacent pixels often have similar colors (called continuous tone). The blue sky is many slightly different colors of blue, you can see that here. In a graphic image, the sky would be exactly one color of blue. And of course, scanned photographs are typically very much larger than 32x32 pixels.