I like to make collages with multiple photos, I think they tell the story and often give each other a context.
I use Blogstomp to make my collages, and find it easy to use and love the results (and I get absolutely nothing for saying that, they don’t even know it).
However, there are two pages that offer free collages:
Pixlr is easy to use. The result is OK, but I find that it is a bit limited.
Canva has a free basic editor that is a little bit harder to use but more versatile, and you can upgrade to get even better results.
A collage can be a diptych with only two photos, or it can be as many photos as you like.
There are a few things to consider when you choose your photos, if you want the collage to actually tell a story and not only be a row of photos. I always try to get a mix of landscape and portrait orientation (horizontal or vertical), and I always try to mix details with overviews, and action with portraits.
If there is a “timeline” (beginning and end) I try to make that easy to read – normally we start to read at the upper left corner and finish at the lower right corner. Think comic book.
Most of the time I just try to get the “perfect look”. I think about the composition, and try to keep the viewer “active” and lead the eye from photo to photo but never out from the frame. I also try to keep nice lines, and colours that o together. The photos should look as a coherent collection.
When you try it yourself – change places of your photos until it feels “right”! It can take some time, but it’s worth it.