There are 50 illustrations (posters) to collect. They appear on your map/minimap as a poster icon. You can buy a map from a shop to help you with finding them. I am including the identifying numbers of each and which borough they are found to help you if you need to find ones you are missing. To check which ones you have found, go to Main Menu > Database > Encyclopedia > Collectibles > Illustrations. The video below is for visual reference and does not go in numerical order by the list below. The list is in the order of the game's collectible list. I added a time list below the video for you to pinpoint a location.
City of London (10) - #1, 2, 3, 4, 5, 6, 7, 8, 9, 10
Lambeth (6) - #11, 12, 13, 14, 15, 16
The Strand (8) - #17, 18, 19, 20, 21, 22, 23, 24
Southwark (7) - #25, 26, 27, 28, 29, 30, 31
The Thames (4) - #32, 33, 34, 35
Whitechapel (5) - #36, 37, 38, 39, 40
Westminster (10) - #41, 42, 43, 44, 45, 46, 47, 48, 49, 50
video courtesy of COG Inc on YouTube
Whitechapel = 00:01 - 01:36
City of London = 01:37 - 05:00
The Strand = 05:01 - 07:26
Westminster = 07:27 - 10:28
The Thames = 10:29 - 12:10
Southwark = 12:11 - 14:06
Lambeth = 14:07 - 15:30