Vector graphics can be drawn directly in LaTeX using specialised packages. This improves the quality of simple designs and allows you to add mathematics to an image. The link below
provides examples of drawing sketches, graphs and charts directly using LaTeX code. It has been written by T. Morales de Luna of Universidad de Córdoba.
Useful Vector Graphic Tools for LaTeX Users
Feynman diagrams can be drawn with the TikZ package or the feynMF package. TikZ is a more general package that can draw any basic shape. FeynMF is specifically designed for Feyman diagrams. A very minimal introduction to TikZ written by Jacques Crémer of Université de Toulouse gives you the basics of TikZ but this fantastic tex document by Flip Tanedo (currently at University of California) gives you multiple examples of using TikZ to create Feynman diagrams. If you wish to use feynMF then Drawing Feynman Diagrams with LATEX and METAFONT written by Thorsten Ohl, Universität Würzburg will provide you with many practical examples.
There are many mathematical symbols and it is impossible to remember all of them. Therefore a cheatsheet is a useful reference sheet that allows you to look up the LaTeX code you need to create each symbol. There are also Apps available for smart devices that allow you to draw the symbol that you want and the App will find the LaTeX code that you need. One example of these from the Google Play Store is Dextify.