You can go to this location: administrator/components/com_spproperty/models and in property.php file from line 131 there is a function old2new
which is maintaining the order of the photos. Sorry, we do not provide customization support.
If you do not want to go to the code, then follow the sequence I shared while uploading photos. If you miss order in any property, then you can change it from the database too. Go to xxxxx_spproperty_properties
table and in the specific property row, find the gallery column. Then change the order there. The same sequence is applied here too. The last one is the first image on the slider, then it continues from the first image to the second last.