The only way to do that is to up-size the images using software like Photoshop or GIMP (a free software). However, the images still may look pretty bad. I would definitely suggest finding (or creating) images that are 300ppi and the size of the intended image on the playing cards.Photo Resolution?
I like your gift idea, but you need to find higher resolution images to start with.
Any adjustment of the resolution of the images you have will not help. They would just be blurry or distorted. (Higher resolution means more pixels. Some software tries to create pixels, but you can't really make something (good) from nothing...)
Most internet images are kept small with low resolution to make the web pages load faster. Sometimes they also want to prevent other people from using the images for free.
Look for a web library of free clip art. Sometimes these have nice photos in a good size. Or look for a library of pics for sale.
Good Luck