Different professionals and illustrators use different approaches for planning Illustrations. When planning your Illustrations, consider the kind of communication product you're producing--technical report, journal article, magazine article, presentation, Web site, book, or poster session. Keep in mind that your Illustration needs to fit the respective format.
Design the Illustration to fit the communication. Don't copy a table from a journal article and used it as an overhead transparency. Seldom will the text be larger enough for the audience to see. You'll need to redesign, and perhaps simply the table when presenting the information as an overhead transparency.
While color photographs can be converted to black and white photographs for printing, they often lose quality and produce inferior black and white photographs. If the final Illustrations is a black and white photography, shoot black and white negative, have black and white prints made and submit them with your report or article.
Simply, the best Illustrations are designed for the publication, targeted to the intended audience, and follow the publication's style.