Depending on the image you want to enlarge, scaling it can be tricky and yield poor results. If the image is stored as a bitmap, you can usually only scale it a little before it starts to look jagged and unrecognizable. Simple, two-color images, such as silhouettes and GIFs, hold up fairly well when scaled, but photos don't contain enough pixel data to fill in the increased area. You can try converting these images to vector format so you can scale them to any size without jagged edges.
Resize a Bitmap
Open the image in a photo editor such as Photo Gallery, Gimp or Photoshop. These programs include a large, varying range of features -- especially Gimp and Photoshop -- but they all contain a scale tool that resizes bitmaps using an algorithm to spread pixels across an enlarged area. In Photo Gallery, click the "Edit" tab and select "Resize." In Gimp or Photoshop, click "Image" and select "Resize Image." Ensure that the "Maintain Aspect Ratio" option is enabled then enter a new width in the text field. The scale tool automatically calculates the new height when Maintain Aspect Ratio is selected. Scale your image by clicking "OK" then export it to your hard drive as an image file.
Convert to Vector Format
Visit a vector-conversion site such as Online Convert, Vector Magic or Autotracer (links in Resources). Converting an image to a vector file removes any color gradation, creating a posterized image with solid colors separated by vector paths. Click "Choose File" to upload your image to the conversion site, then select "Convert" to create an SVG file from the image. Click the download link to save the file to your computer, then open it in a vector editor such as Inkscape, DrawPlus or Illustrator. These programs support scaling the image to any size, exporting a large bitmap or printing the image in any dimensions. To print the file, click "File," select "Print" and enter the paper size in the dimension fields.