Adding the information about using the "Spoiler" tag here too in case folks are searching Community for how to do it.
After you click Reply on the Weekly Challenge you have solved:
There is a special "Spoiler Tag" icon in the WYSIWYG toolbar, click on that and a Spoiler tag section will be added. Then paste the copied screen shot of your workflow / results inside of that. To upload the actual exported workflow (if you export a yxzp from Designer it will include everything required to run the workflow), click below/outside of the Spoiler area and Browse files to attach to select your workflow file. Click Reply to submit your solution for review.
Sr. Technical Product Manager, Alteryx Engines