Go to My Tests. (You must be logged in to see this page.)
There, look in the top left of your screen for a + New Test button.
Once you've tapped the "New Test" button, follow the steps on screen, starting with adding or selecting a photo.
Note that adding a photo is not the same as starting a test. Follow all the steps until you receive a confirmation that your test has started.
If there is no "New Test" button when you first sign up to Photofeeler, simply follow the steps on the screen.
While looking at your paused test, notice an + Add Votes button in the top right of your screen.
When you tap this button, a menu will appear. Make your selection, then you will receive confirmation that your test is active.
If you're a Karma user, you're restricted to testing one photo at a time. If you've purchased Credits, however, you can test many photos at the same time.
When you have multiple tests running at the same time, Photofeeler spaces out those photos in the voting queue. This way, voters won't see pictures of the same person too frequently.
If you're worried about voters seeing more than one picture, you can test them one-at-a-time. But this is typically only a problem for a specific kind of test (e.g. if I edit my background in one photo and then show the real background in another photo, in which case seeing both photos would "give away" my deception).