Select an image. It can be of basically anything; everything from nature photography to abstract 3D art.
If you want music that fits a certain emotion or occasion, try to find the image that best represents it, and specify the emotion/occasion in the title.
Example: A photo of a beautiful, sunny beach.
Choose a title that describes the image, and emphasize the features of the image that you desire in the music. Prefix it with [MFI], so it becomes clear on the front page that it comes from this subreddit.
Example: [MFI] Happy, sunny beach
To recommend a song for an image, just link to it in the comment section. Specify the genre. It doesn't have to be a song with a topic that immediately relates to the image, as long as it fits the mood.
Example: Dinka - On The Beach [Prog House]