Sorry it took so long to reply. You asked how I made this. For all of my photosets I usually use a template. You can find them in a lot of different sites but I only use templates from welovetemplates. There’s a larger variety, and there are a few tutorials.
I’m fairly sure I used this template for my Lee Min Ho post. You can approach this a lot of ways but if you have a template this simple all you really have to do is match your picture with the dimensions that they have on their template. (I’m just going to assume you already know a little bit about photoshop.) It might make your life easier to copy and paste the template into photoshop.
The first thing you do is figure out which four pictures you’re going to use. For this type of template I would use pictures that are larger in height and lower in width so they will fit properly.
Go to File -> New and enter the same sizes from the template into the box.

You should get a blank box that is the same size as the template picture. From there you can simply paste your picture into the blank box and color it. Or you can color it first and then drag it into the new document. Once you’ve done this with two pictures you must combine them. Since I’m all paranoid about my work I always save first. You can either save them individually, or leave them somewhere so that you can easily pull them up if you need them.
You can do this by checking to see how big the image is. In order to do this you go back to your template picture go to Image -> Image Size and it will show you the dimensions of the picture as a whole. In this case it’s 245 x 330 px.

Therefore you need to hit File -> New and enter the sizes as 245 x 330. Make sure the background contents are changed to transparent.

From here you can either open the pictures that you saved and fit them into the box. Or you can drag them if you still have them up.

Once you’ve finished with one set, do it again with the next set to complete. Make sure you save both of them under .PNG! This will keep your transparency.

A box will appear after you’ve named it, regarding interlace. Just click “ok.” After you’ve done this to both pictures. Open up your tumblr for a new picture post.
Make sure you’ve selected THIS template!

I know the way I’ve explained it might seem a little confusing. If you have any other questions about a more complex template you can message me. I have different methods for more complicated layouts. Hope this helped a tiny bit!