If you are looking for the code to put on your userpage it would look something like this:
[url=https://osu.ppy.sh/u/friend1][img]friend1image.png[/img][/url][url=https://osu.ppy.sh/u/friend2][img]friend2image.png[/img][/url][url=https://osu.ppy.sh/u/friend3][img]friend3image.png[/img][/url]
If you want to put the entire picture on your userpage, instead of just the faces of each of your friends, I would suggest slicing your image with an image editor vertically in-between each of the characters and saving each individual slices as a separate image. The crudely drawn lines in this image explain what I mean:
Each section of this image would be one of the "friend#image.png" images in the codeblock above.
If you want it to fit on your userpage perfectly, you first have to make sure the image is not wider than ~600px and then slice the image into however many slices you want.
You can make much more complicated slices horizontally and vertically for different images, but it gets really confusing to keep track of.
