What is normally at the beginning of the field and tells the computer how to build up the picture including the size and number of colours used? this question is for a crossword me

Normally picture files contain a header before the main section of data, this header tells the operating system/application the format/encoding of the picture (jpg, png, gif, etc), the resolution of the picture, the number of bits used for a single pixel (which determines how  many colours could be used) and other details about the construction of the picture. 

You aren't giving enough information. what kind of "tool" do you mean? if you want an opinion, then jeremy should use a computer to show how his web will look like.