...as the other posters have said. Quick rule of thumb (and your mileage may vary!) is you need 6 megapixels (MP) for an A4 print and pro rata for other paper sizes. You can easily check the resolution of what is on the disk with Windows explorer, just mouse over the file and the dimensions are the pixel size, horizontally and vertically - just multiply them together. Check both what is on the disk the photographer gave you and the files on your computer, if you copied them to your hard disk before sending them to Walmart - Windows should have copied them as is but some photo album software can compact the pictures in various ways.
If the image size in pixels, is in the right ball park, the problem could be due to a few sources but most likely the compression ratio. For example (and again, your mileage may vary), my Fujifilm consumer digital camera is 3 MP and stores images in JPG format at an 80% compression ratio (the files are 6-700KB). These print just fine on A5 but show some pixellation/noise at A4. They display just fine on a 22" PC screen, since anything above 72 dpi is better than the screen resolution. I can take the JPGs from the camera and compress them down to about 45% with Irfanview (great simple photo manipulator and free!), the files reduce to about 75-90KB and they will still display perfectly but will disappoint on a photo printer.
I would expect a professional photographer to use RAW format in the camera, available on all professional and most prosumer models. You do need more than the average photo package to handle RAW files, though and so a lot of professionals will also include a copy of the pictures in some other format - often JPG. Apart from this being the most common compressible format (RAW files are huge), I'm not sure why they use JPG, which uses a lossy compression technique, rather than, say PNG which uses non-lossy compression. As the names imply, lossy compression throws away some of the pixels and once they are gone, they are gone; non-lossy means that when the files are re-expanded for printing, all the pixels are retained. The downsize is that PNG files tend to be bigger than JPGs but much smaller than RAW. Most JPG image manipulators use an 80% compression ratio, which will lose pixels during the editing process.
If the disk has multiple formats on it, try one of the non-lossy ones, RAW (if you have anything to handle it) or PNG or BMP if they happen to be there. If you've only got JPG, if the printed images aren't good enough, you'll need to talk to the photographer about getting some prints from her. She'll likely have a professional quality printer and should be able to produce the sparkling prints you want.
As dxjanis says, the simplest way to find out what the actual quality of the images is, is to take the original disk (make a copy to keep first, just in case!) along either to Walmart or to a serve yourself outlet (most office supply chains will have one) and print one image directly from the original disk. If that turns out to be the quality you want, then the problem lies in something either you or Walmart did in the transfer and printing process. Print any more images you want from the original disk. If a direct print still isn't good enough, your only recourse is to talk to the photographer about it.