I know that you lose quality going to a lossy format.
Examples: flac > mp3 ogg > mp3
But do you lose quality going from it?
Example: mp3 > flac
Bonus question: What about images?
Example: jpg > png
When you convert from FLAC to MP3 (or whatever formats), the converter has to essentially re-record the file. It would be like taking a video of a video, or taking a photo of a photo; of course you're not going to maintain the quality of the original.
However, when you convert from MP3 to FLAC, you're simply decompressing the MP3 and taking the raw sound of whatever is there. No quality loss. Of course, as James Polley pointed out in the comments, you can't gain quality out of nothing in this way.
This applies to audio, video, images, etc.