You can increase the resolution of a digital image file, but you will lose image quality by doing so. If you are using high resolution images and need to increase the image size, you may try to enlarge the image no more than 10-15% of the original image, if you exceed that percentage, the quality lose will be very noticeable. Here is a video tutorial that explains how to resize a picture: https://www.youtube.com/watch?v=oZAGC3ptdR00