This is the trickiest one and will get you doing some html.
First select the "Layout" tab. Then click on "Edit HTML." This will launch, suprise, suprise, the html editor. Now scroll down through the html of your blog and find this (click on this image to zoom in on the text):
All you need to do is make note of the width.
My blog's width is apparently 410px.
When you upload your pictures, you will need to make sure that they are less than half this width and that both are the same width as each other (e.g., 200px).
Following is an example.
What is better than one Lady Gaga? Two Lady Gagas!
Here is the picture we will use:
And this is the html code for the picture:
STEP 1: Import the picture.
STEP 2: Resize the picture.
GaGa's picture is wider than 200px. GaGa is 254px.
Let's slim down GaGa! I replace "254px" with "200px" so that she can fit twice.
And erase the height value (it is also 254px because our picture is square):
And we get this:
STEP 3: Let's put two photos side by side.
Immediately paste the html for the photo directly after it on the same line.
Paste this immediately after the photo's html on the same line:
Hit the Return button.
Repeat STEPS 2 through 3.
STEP 4: Paste this on the next line.
Which gives you this: