From the Abraham Lincoln Presidential Library website:
"Almost 3,000,000 soldiers served during the Civil War. Over 285,000 of them were Illinois soldiers who fought for the Union. In the 1860s, photography was becoming a popular way for average families to capture their images for posterity. Soldiers often had a "likeness" taken before they marched off to war — or they sent one home later, to a sweetheart or parent.
As part of its extensive photographic holdings, the Abraham Lincoln Presidential Library owns around 7000 images of Illinois "Boys in Blue." As part of a grant that the Library received, information about these images (and several hundred more that belong to other libraries and museums across the state) was entered into a database. This database lists the soldier's name, regiment, town of residence, and other information. At this point, the photos themselves are not available on the web. If you would like to see an image, contact the institution that is listed as the owner."
Click "Boys in Blue Database” in the right-hand navigation bar.