1. If the image is to tall/wide it will be scaled to fit into your browser. This happens via CSS. Photos won't be compressed or scaled, Lychee keeps their original quality and dimensions. You can click the 'Full Photo' button on the top right to see the file.
2. Nope. Not at the moment.
3. Does this happen only sometimes or always? Which browser and OS are you using?
I also have the same problem (3). It happens when using chrome 32 on windows 8.0 with full screen mode enabled. The scroll bars appear when I open an image and click the right arrow on my keyboard to switch to the next. For me it doesn't happen if I don't navigate to another image or if I'm not browsing in full-screen mode (f11).
2. Nope. Not at the moment.
3. Does this happen only sometimes or always? Which browser and OS are you using?