RESOLUTION
Problem:When the customer tries to upload an image which size is over 512 KB, it disappears.
Solution:
There is a limit of 0.5 MB for files that users are allowed to upload to SiteStudio by default. This limit is set because a part of SiteStudio (called ImageMaker?) that deals with images, i.e. generates thumbnails of images--uses more than 128MB of memory to produce the thumb for a JPG file with size more than 0.5MB.
You can increase that limit by setting (in bytes) the MAX_IMAGE_SIZE variable in the masonry.properties file (after that the "cut picture size in half if >500K" switch makes sense). E.g., to set limit to 1MB you need to add in the masonry.properties:
MAX_IMAGE_SIZE = 1048576
Also it's would be better if you allocate more memory for imagemaker process on your system. To do that, you need to modify imaker.sh file: in the 5th line change -mx128m to -mx256m or even -mx512m if you wish to allocate 256 or 512MB correspondingly of RAM for imagemaker.
After changing that you need to restart SiteStudio:
./ss.sh start
./ss.sh stop