An illustration, in the sense of the question, is an example which is used for elucidation or comment. It might also be the explanation of a text by means of a print, a figure, a drawing, a picture or an image but it is not this aspect which it is wished to consider.
The use of illustrations is very useful to help do understand a teaching or a message, or to emphasize an aspect of them, and is commonly done in sermons for that purpose.
In order to see how far it is valid, let us first see how illustrations are used in the Bible, for we find plentiful illustrations in the books of the Old and New Testament.
Bible illustrations encompass what is seen in nature, historical events, and allegorical narratives which transmit an indirect message, or a spiritual or moral precept by means of comparison or analogy, called parables.
In the books of Job, Psalms, Proverbs, Ecclesiastes and Songs of Solomon (called poetic books), we find the following types of illustration:
emblematic parallelism, where a figure of speech is given in a first line to illustrate the contents of the second (Proverbs 11:22, 42:1, etc.),
literary figures to make comparisons. They may take the form of similes (Psalm 5:12, Songs of Solomon 2:3a, etc.), metaphors (Psalm 84:11, Songs of Solomon 4:12, etc.), anthropomorphism, where shapes, behaviour and thoughts characteristic of the human being are attributed to God or other spiritual beings (Psalm 11:4, etc.), or even those of animals, zoomorphism (Psalm 91:4), and personification where human words and sentiments are attributed to inanimate objects, animals, etc. (Psalm 96:11,12, Proverbs 8:12).
In a more ample manner, all the history of humanity and of the people of God found in the Old Testament serve as illustration for a better understanding of the relationship between God and humanity, God's ways and human nature. Let us see some of the innumerable cases:
The priesthood, the tabernacle, the rituals, "(the priests who offer the gifts prescribed by the law) serve at a sanctuary that is a copy and shadow of what is in heaven. This is why Moses was warned when he was about to build the tabernacle: 'See to it that you make everything according to the pattern shown you on the mountain'" (Hebrews 8:5 - NIV).
The destruction of Sodom and Gomorrah and surrounding towns who gave themselves over to sexual immorality and went after strange flesh (homosexuality), are an example to those who thereafter would live ungodly lives (2 Peter 2:6, Jude 1:7).
The rebellion of the children of Israel in the desert, which prevented them from entering the promised land (Hebrews 3:7-11).
The affliction and patience of the prophets who spoke in the name of the Lord (James 5:10).
The New Testament is also abundant in examples which are useful as illustrations: the apostle Paul is an illustration of the mercy and long-suffering of Christ (1 Timothy 1:16); he is also the illustration of a faithful and hardworking believer, facing all vicissitudes for love of Christ (Philippians 3:17); the members of the church in Thessalonica illustrated the election of God, having received the word in much affliction, with joy of the Holy Spirit, becoming followers of Paul and of the Lord (1 Thessalonians 1:7).
Our supreme Master, the Lord Jesus, made much use of parables, of which not less than thirty three are found in the Gospels. They illustrate teaching concerning the Kingdom of God, service and obedience, prayer, neighbourly care and humility, wealth, they explain the Gospel as regards the love of God, gratitude, and warn about the future concerning His return and the judgment of God.
In their great majority the parables He said are not necessarily historical facts, but concern situations which can happen, in this way illustrating parallel truths. He explained to His disciples that he enunciated His teaching through parables in order that those who wanted to listen to Him might understand the mysteries of the Kingdom of Heaven (Matthew 13:10-17).
We also find parables in the Old Testament, such as the one Nathan used to awaken in David his conscience to his sin (2 Samuel 12:1-4), that of Jotham (Judges 9:7-15), and many others.
We conclude, therefore, that fictional stories can be used as well a facts as illustrations for elucidation, as in the parables. It is not lawful to us, however, for the sake of illustration, to tell an invented story or to repeat something we have heard from a doubtful origin, pretending that it is true fact. If used for illustration, their fictional or doubtful origin should be made very clear to the audience.
Given the great volume of illustrations given to us by the Word of God itself, it is generally not necessary to make use of our imagination or to search in another source for material to help to clarify the Bible teaching. If we keep to the illustrations in the Holy Scriptures, we will avoid the risk of badly or incorrectly illustrating what they teach.
Finally, better than any verbal or written illustration, we are admonished to be examples, or living illustrations to the others, believers in word, in conduct, in love, in spirit, in faith, in purity, in good works, in doctrinal integrity, in reverence, in sincerity, in sound speech that cannot be condemned (1 Timothy 4:12, Titus 2:7). Let us make this our priority.