What is a Joiner???
Easiest to go to google, enter ‘Hockney's Joiners’ and click images... you will get the idea immediately
A Joiner is literally a collage of as many photos as you can stand to put together, assembled in a way as to represent a whole image, each photograph being a small part of the bigger picture. For instance if you wanted represent a full facial portrait you would take several pictures of the eyes (or each eye), ear, nose, forehead, hair, cheeks, chin etc. Once you have all the images to make up the face you then pile them up in a fashion that the face becomes apparent. Lots of small detailed images put together to form the whole image. Done well it looks amazing.
Before you do anything in PP, ALWAYS make a copy so you don’t lose anything should you make a mistake 

How to do a Joiner;
To begin with I am just going to say that as with everything in life, the more you put in, the more you get out! There are several ways to do a Joiner or a joiner effect, some are easy and some are downright confusing... but certainly not Hard!
1. the simplest....! take one image and upload to bighugelabs.com/hockney.php and click on the Hockneyizer button and everything is done for you. It’s limited but real easy and fun the hardest part is choosing the right image.
2. The hardest; take loads of images of the details have them printed and lay them out as Hockney does and take a picture of the resulting image. When you put the prints together they do not have to be accurately positioned, in fact it looks best if they are not! Hard work, a lot of it and relatively expensive but my goodness it’s extremely satisfying 
3. Take loads of pics and put them together as a montage in an editing suite- It’s similar to the methods above but you do in fact have more control... you can mess with each individual image in terms of it’s relative formatting, landscape, portrait or even angled. Hard work, a lot of it and not so expensive and still extremely satisfying  Here is just the best Tutorial you will find anywhere for this technique... www.youtube.com/watch?v=F3q59aeDYqc and another for polaroid style www.youtube.com/watch?v=ZY6A826F2y0 While these are specific for Photoshop apps this one is for Gimp.. www.youtube.com/watch?v=06J0sOZG9Ko but to be honest Gimp is a bit slow and the tutor is not very clear. But by watching all three you will get a real idea and feel for the techniques.

My own tutorial is published as 7 individual images P1-P7 for you to follow and is designed in a walk-through format.- it’s based on Pixlr (Go to pixlr.com/editor it costs nothing and everything is in the ‘cloud’) and it could not be easier... Pixlr is Very capable and is quite similar to Photoshop.
Upload your image or a copy of it just to be extra safe! You have a choice from where to gain your image, I clicked from my computer but you can direct it to your flickr/ipernity account i believe! This is the first page with the others below, click to follow through to the image... if you need any help just ask- send me a mail

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