You can upload any files or documents when you are creating your brief.
Next to the "Logo Design Brief" text you can then click "Edit" to change information. You may add or delete material. Remember to click "Save" when you are done.
Please note that you need to be clear about the usage of any reference material. There are two kinds of reference:
Reference You Own: this is reference that is 100% your property or that of your client. eg photos, artwork etc. Designers may incorporate such imagery into your logo as a derivative work.
Reference You Do Not Own: this means that Designers may not use the image in any way that the copyright holder may view as violating his or her usage right. eg. car dealers or charity events using a parent company or sponsor logo.
If you represent the copyright holder, you should explain that clearly and provide the relevant vector file for Designers to use and explain any limitations or restrictions on how it may be used or displayed.
Stockart may never be incorporated into logos. Such imagery usually has strict usage rights as defined in the EULA for the image. If you have concerns, check the Licensing section on the site that is hosting the image you plan on using.
If you must edit the brief during the contest, please do not make posts using concepts from other Designers that did not originate with you.