Are there any pics online of the old courthouse?
Oh, yes.
http://courthousehistory.com/gallery/states/illinois/counties/alexander
Pictures #3-5 are the demolished courthouse. Curiously, another even older Alexander Co. CH located in the hamlet of Thebes still exists (which is on the Mississippi — Cairo’s downtown is on the Ohio). I took some pics of it en route to Cape Girardeau back in 1992. They wisely built it up on the high bluffs, whereas the old town of Thebes down on the riverplains got flooded and there’s nothing much left there.
The old federal courthouse still stands (which became a Custom House): https://www.flickr.com/photos/courthouselover/3388806506/in/set-72157625528948313
Also across from the new (but 51-year old) courthouse is the old library: https://www.flickr.com/photos/courthouselover/3388806498/in/set-72157625528948313
This was the Halliday Hotel where Grant had his HQ during his time there in the Civil War: http://www.legendsofamerica.com/photos-illinois/HallidayHotelCairo.jpg
It unfortunately burned in 1942. It’s an open field today:
https://maps.google.com/maps?q=cairo,+il&ll=36.998765,-89.149783&spn=0.000034,0.021136&usg=__hStcENI7i5cl4AWsgs_HOXA2GgI%3D&pjf=1&hnear=Cairo,+Alexander+County,+Illinois&t=h&z=16&layer=c&cbll=36.998743,-89.161474&panoid=m5UvMGJBJjRiNwquBn6Ljw&cbp=12,160.54,,0,2.3
