Picturando is a photo editor online that works on mobile and desktop. You can easily add your logo to to photos and watermark images.
Instructions: it's easy, click to "Open" and select "Background Image" to upload your photo. Then click again "Open" and select "Overlay image" to add your logo. Now you can simply drag/drop/resize/adjust. When ready, click "Download" icon.
Tip: use a .png with transparency as overlay image, to get better result. The editor has many other function built in (like add text, add emoticons, add shapes, stickers, etc.)