Microsoft Paint is a free raster graphics editor that lets you create and edit photos and other images and save them in a variety of formats. The tools provided in Paint are relatively simple compared to more sophisticated products like Adobe Photoshop. Since the concept of layers isn't available in Paint, you can't overlay or merge pictures, but you can combine two pictures by displaying them next to each other.
Inserting a Picture in Paint
To combine two pictures, start by creating a new blank picture using the New command on the Paint tab. If you want your finished combined picture to have specific dimensions, set them now with Resize. Setting the zoom level so your new blank picture is entirely visible will make it easier to place the pictures next to each other. Insert the first picture by bringing up the Paste menu on the Home tab and selecting "Paste From." A file box opens and you can specify an existing picture file. Use your mouse to drag the picture to position it and the resize handles in the corners of the picture to set the size. You can also rotate or flip the picture or trim it with cropping.
Put Two Pictures Together
Once the first picture is sized and positioned, you can open another picture using Paste From. Resize and move the picture so it's side-by-side with your first picture, or in whatever layout you want. If you need to modify the size or location of either picture after it's unselected, you won't be able to select it directly again because it's now part of the overall raster image. Go to the Select menu on the Home tab and enable "Rectangle Select," then drag the selection rectangle around the picture. You can now use the resize handles to change the picture size or move it by dragging.
Save Combined Pictures
When you place two pictures side-by-side in Paint, you can leave a small amount of white space between them, or you can place them right next to each other so they appear to be stitched together. You can add more pictures using the Paste From command to create a collage. Once you're happy with how the combined pictures look, use the Crop command to remove excess white space around the pictures. Then use Save As to name your new picture and save it as JPEG, PNG, GIF or one of the other supported file formats.