You can upload Your own files to FashLab.
The files You upload to FashLab are only available under Your account. They are only for Your private use and other users don't have access to these files.
They become available only for the production house if You would like to order it.
So You do not have to worry that anybody else could use Your files.
You could also filter Your own files from public files when using filtering.
FashLab also wants to remind You that You are responsible for all IPR (intellectual property rights) for these files. Please make sure they do not violate anybody's IP rights. As images downloaded from internet without asking any permission is not allowed. Photos and illustrations are usually under copyrights.
However there are many platforms that sell or offer images (photos, illustrations etc) with copyrights like shutterstock, gettyimages, hiclipart, dreamstime etc.
Make sure You read their Copyright policies.
Read more
You can read more about Copyrights here.