For the endocrine system, it might be hard to find illustrative pictures to memorize things. You just need to understand that the endocrine system involves communication between cells in a slow, indirect, and nonspecific way. This just implies that hormones pass through the blood and circulate through most of the body, affecting many types of cells. Contrast this to chemical synapse (nervous system) and electrical synapse (cardiac cells, for example) which are fast, direct, and specific. Other than that, the big thing is to know all of the important hormones, their targets, effects, and regulation. Have you tried flashcards?