I can't apply my skin while playing offline (without internet)

I started playing without internet, but I noticed that the default skins are always 'Steve' and 'Alex', and I can't import my .png skin into Minecraft.

The options are locked, and I can't select my skin, Steve's skin, or Alex's skin. I don't know if it's a bug, but it's frustrating because I record my gameplay using my skin, and now I can't.