Wendy is right. I hope you don't mind my specifying a bit...
I first open both images in Photoshop, and then I check whether they are both the size I want. If not I adapt. (check whether both are at the same % also).
Then I Look at the one I want to add to the other and decide whether it has to be beside, above or under (vertical) of the other. I also note its dimensions in File Menu>Image Size. If I want it beside the other one, the width is important. When they have to be vertically arranged it's the height that I note.
I go back to the other image and choose Image>Canvas size, and enter either the width or the height in the appropriate box so as to make the canvas wider or higher. Then I go to the square with the arrows and click on the arrow where the image I'm now working with should be. Example: if I want the other pic on the left, I click the arrow on the right, pointing to the left, so that what I add will be added on the left. Click OK.
Now I see that I can see both images in the workspace (In Winwdows the middle icon top right), take the Arrow tool (top right in the toolbox) , click on the image I want to import and drag it unto the other one with the extra space. It will open on a new layer as you can see in the Layers palette.
Like Wendy wrote, you now can drag this image-on-the-layer on the empty space that you prepared for it. You can also nudge with Ctrl+arrows on your keyboard.
When it is perfectly placed, you can now click on the right pointing arrow/triangle at the top-right of the layers palette and choose Flatten Image. If it's for the web, you can save for web and choose jpg or gif.