Make sure that the logo you are uploading do not infringe on any copyright, property right, trademark or any other applicable right.
This is NOT a famous brands stock but a free logo templates site so upload stuff that you and only you have created. You can for example submit left overs options of works you have done, junk files you won't use, study creations, etc.
We highly encourage to share vector files, we may accept also PSDs but of course for logo design community prefer scalable artworks. Remember your design is showcasing your skills, include a link to your site in the submssion and also recommended in your actual logo file.