I just tried it with GarageSale 7.0.16 on High Sierra and the media browser is working fine here.
Did you make any changes to your system or to the Photos app recently?
If you open the Photos app, does it show your photos or is it empty?
In the Photos preferences is there a photo library selected?
Also, is the button “Use as System Photo Library” in the same preferences greyed-out or is it clickable?
Your Photos preferences looks fine to me.
Can you please check the System Preferences > Security > Privacy
and select “Photos” in the left table there. Is GarageSale listed on the right side?
That looks fine, too.
Unfortunately, I am a bit clueless here. Maybe it has to do with the iPhoto library. Could it be that the iPhoto library show your photos from the “Photos” app, too? (in GarageSale’s Photo browser)