Prepare your logo file
The best file is a PNG with a transparent background, the version a designer supplies for use over photos. Make sure it's large enough: a file at least as wide as the logo will appear on your biggest photo stays sharp, while a small one goes soft when it's scaled up. In practice, a PNG 1,000 to 2,000 pixels across covers almost any photo.
Keep a light and a dark version if you have them, so you can use whichever stands out on each photo. A simple mark or icon also reads better at small sizes than a full logo with a tagline.
If all you have is a JPG on white, use it. The white around the logo, and inside letters such as O and e, is made transparent automatically. Transparent space around the logo is then trimmed, so Size refers to the logo itself.
Size, opacity and position
Most logos work best as a small mark in a corner. Use the Position grid to snap it to a corner and the margin to keep it off the edge, or drag it anywhere on the photo. Size and margin follow each photo's shorter edge, so the logo looks the same on portrait and landscape shots.
As a starting point, make the logo about a tenth of the photo's shorter edge and set Opacity between 60% and 80%. Go smaller and fainter on portfolio images where the photo should lead, and larger or stronger on images you expect people to reuse.
Rotation turns the logo anywhere from -180 to 180 degrees, but most logos look best level. To make the logo harder to remove, place it near the subject, or repeat it across the whole image, which is a Pro feature.
Logos on light, dark and busy photos
A logo that stands out on one photo can vanish on the next. If your set mixes light and dark shots, a version with a thin outline, or one inside a solid shape, reads on both, so you don't have to swap files between photos.
Over busy backgrounds such as leaves, crowds or patterned fabric, a solid badge reads better than a thin wordmark. Raising the opacity helps a little, but contrast does more.
A logo and text together
The free plan gives you two layers, enough for your logo plus a line of text such as your website, handle or © notice. Layers are drawn bottom to top, and each has its own position, size and opacity. Pro allows up to six.
To make the two read as one unit, put the text just under or beside the logo and give both the same opacity. The Logo corner template is a quick place to start.