Nuyt, I looked at the St.Chamond 70 mm, not the 80 mm, and did see some resemblance to the gun on the photo, but was not sure. Venezuela had 8 St.Chamond 70 mm howitzers. I now have the idea the photo was taken in Venezuela and not in Paraguay. Maybe the photo was taken by a Paraguayan soldier in Venezuela, there are also the words Simon Boli(var) on the building, so it's more likely Venezuela.
For photos St.Chamond 70 mm howitzer see: www.lovettartillery.com
So either a St.Chamond 70 mm howitzer or 80 mm mortar?
I go for the 70 mm, with pneumatic tyres and a new shield, of the Venezuelan Army and nothing to do with Paraguay (except for the soldier).