The earliest picture postcards, I have found to date are from around 1898, mainly multi-view's as shown here. However earlier cards do exist particularly from London as below (1897) and abroad much earlier (1870's).
In 1894 - Britain's produced it's first Picture Postcards.
From 1 September 1894, the Post Office allowed postcards published by others to be posted. A halfpenny adhesive stamp was to be added to these cards before posting.
Several manufacturers produced cards.
The first publisher to include pictures to the cards is believed to have been George Stewart of 92 George Street, Edinburgh.
From 1895 onwards, a size of 4.75 ins x 3.5 ins was adopted for postcards. These were known as Court Cards. The address was written on one side. The reverse bore a small picture leaving sufficient space to write a message
Early Harwich & Dovercourt Cards ( 1899 - 1903 )
From 1899 onwards, the standard size of 5.5ins ins x 3.5 ins, already in use in other countries, was accepted in Britain. The address and nothing else, still had to be written on one side of the card. The other side being for the picture and message. In many cases the picture covered most of the card, leaving little room for the message.
Among the earliest cards were greeting cards from Parkeston Quay, the earliest card had plain backs and only the stamp and address was allowed to be written. Any messages had to go on the front of the card.
Early publishers here include Edgar Schmidt of Dresden-Saxony (Germany).
Moving away from Multi-view we start to see the more familiar "scene" style cards, although the former has stayed popular throughout history.
Queen Victoria was the longest reigning British monarch in history, ruling the United Kingdom from 1837 to 1901. Her death on January 22, 1901 at age 81 was mourned around the world and signalled an end to the Victorian Era.
These early postcards retained the plain backs with only the stamp and address allowed. Any messages still had to go on the front of the card.
Edward VII (Albert Edward; 9 November 1841 – 6 May 1910) was King of the United Kingdom and the British Dominions and Emperor of India from 22 January 1901 until his death.
The eldest son of Queen Victoria and Prince Albert of Saxe-Coburg and Gotha, Edward was related to royalty throughout Europe. Before his accession to the throne, he served as heir apparent and held the title of Prince of Wales for longer than any of his predecessors. During the long reign of his mother, he was largely excluded from political power and came to personify the fashionable, leisured elite. He travelled throughout Britain performing ceremonial public duties and represented Britain on visits abroad. His tours of North America in 1860 and the Indian subcontinent in 1875 were popular successes, but his reputation as a playboy prince soured his relationship with his mother.
In 1902 the Post Office changed its rules and allowed pictures to appear on the front of postcards and messages and address both to appear on the back.
The message was to be written on the left-hand side of the back and the address on the right-hand side of the back. Great Britain was the first country to allow this practice.
From around September 1902 onwards, postcard manufacturers began to issue cards with a line drawn down the middle of the back to show where the message and address should be written. These cards soon replaced the earlier ones with 'undivided backs'.