First of all I should make you aware of BeScouted a photography community linked to steemit. With respect to contests, the following users all organize contests, some daily, some weekly and some bi-weekly:

• @juliank (daily - you enter automatically by using the proper tag - different categories)
• @czechglobalhosts (daily - enter in the comments of his contest posts - nature scenes)
• @jasonrussell (weekly - use #portraitcontest - portraits)
• @customnature (weekly - use #aswcontest - "abandoned shit")
• @daveks (weekly - use #bwphotocontest - black&white)
• @killerwhale (weekly - use #killerpix - miscellaneous)
• @brumest (weekly - use #monomad - black&white)
• @jamtaylor (weekly - use #steemitphotochallenge - miscellaneous)
• @kus-knee (weeky - enter in the comments of the contest post - "mundane art")
• @timsaid (weekly - use #mypictureday - post about a day in your life)

Good luck if you decide to enter a contest, some give quite decent prizes!