There is a different contest everyday. These contests are repeated every week. Contest with photos of Czech Republic are everyday though (at least this is my observation). It's very simple as you just need to wait for his post announcing the contest and post your photo in the comment section under that post. You can check the one from yesterday here. The contest ends on Wednesday 8 AM CET. After that Tomas goes through the entries, upvote them and eventually picks the winner.

I hope this helps.