A few users seem to experience some problems with file dialogs under High Sierra [:(]
As I don't have any issue on my side, it's difficult for me to investigate. Some users reported that the issues disappears for a while after a reboot. A more reliable solution is to drag and drop the rendered image on Preview application, and save it in Preview.

What version of Sweet Home 3D do you run (installer version, Java Web Start version or Mac App Store version)? In case you know it, is your disk formatted in APFS or HFS+?