PicArtia is an online tool that you can use to create a Photo Mosaic. A PhotoMosaic is a picture that has been divided into rectangular sections, each of which is replaced with another photograph of average color. When viewed from afar, the individual pixels appear as the primary image, but when closely examined, it reveals the image(s) content present in those pixels.
To use the tool, upload your main image or the master image. You can upload directly from your hard drive or grab an online photo (JPEG files only). After uploading you photo, just select from their list of galleries which will be used to create your mosaic. You are allowed to adjust a few settings before you head over and create your final image. The vertical and horizontal slice of your master image can be altered and the cell size (width and height) of the gallery images you selected can also be adjusted. Just be advised that the resolution of your final image will also be altered if you tone down your settings. If you want a high resolution image, just stick with the max settings.
To download the image, you must provide your email address. A link to your final image will be emailed to you along with the code which you will need to download your mosaic. The service is free for A5 and A4 paper sized images. If you want larger sizes and higher image resolutions, you can order them for $2.19-9.19, depending on the image size.