Surely you have a tool that makes thumbs. Use it to make the images larger. I use IrfanView and use TCG images all the time. You can select a batch of images, set the image size, and they all come out whatever size you select. You can also select the compression you want and it's all done at the same time. The best part...IrfanView is free.

You can get it here:
http://www.tucows.com/preview/194967
If you don't like Tucows do a search on google, there's plenty of download sites listed.