From the beginning.
Set your Vegas Project properties for 720 P (assuming you are using a 30 fps frame rate) you should be able to find the correct set up already as a template, otherwise take the closest thing and customize it. Adjust the audio tab to the best match to your sound track.
Next add your sound track to the Vegas Timeline.
Next add your photo, you may have to stretch or resize it to match the length of time of the music.
Then right click on the photo in the timeline and choose properties, click the box that has 'reduce interlace flicker' (whether this will help a still photo, I am not certain, but it won't hurt).
Now, play it back in the viewfinder, and adjust the master volume on your sound track so that it does not exceed 0 dB or under -.5 if you want to allow a bit of a safety margin.
Also, you can fade in or out either or both the sound and image tracks, or add other effects.
Finally, engage the 'render as' button and choose mp4, 720p. Click 'custom' to tweak for the 'best' and set the bit rate somewhere between 5 MBS and 10 MBS, you may want to tweak other items as well. If you are not sure, try different renders to see what works best for your work. Suggestion: If you want to try different renders, make a short 20 second version as 'test' project of the original material so you won't have to wait so long for each render.
I suggest mp4 as per YouTube recommendations, also I have uploaded 720p mp4 with audio to YouTube and can testify that this will work fine. MP4 is optimized for internet streaming so you will likely get the least amount of stop/go streaming issues with this format.
YouTube's recommendations printed here for your convenience:
Encoding Video For YouTube: Advanced Specifications
We suggest the following format for high quality uploads to YouTube. Note that these are advanced encoding specifications, and we have simpler encoding instructions available elsewhere.
• No Edit Lists (or you may lose AV sync)
• moov atom at the front of the file (Fast Start)
Audio Codec: AAC-LC
• Channels: Stereo or Stereo + 5.1
• Sample rate 96khz or 48 khz
Video Codec: H.264
• Progressive scan (no interlacing)
• High Profile
• 2 consecutive B frames
• Closed GOP. GOP of half the frame rate.
• Variable bitrate. No bitrate limit required, though we offer recommended bit rates below for reference
• Color Space: 4.2.0
Frame rates should match the source material
• For example, content shot in 24fps should be encoded and uploaded at 24fps. Content recorded at 30fps should be uploaded at 30fps.
• Content shot in 720p60, should be uploaded at 720p60
• Content at 1080i 60, should be deinterlaced, going from 60 interlaced fields per second to 30 progressive frames per second before uploading.
YouTube uses 16:9 aspect ratio players. If you are uploading a non-16:9 file, it will be processed and displayed correctly as well, with pillar boxes or letter boxes provided by the player. If you want to fit the player perfectly, encode at these resolutions:
• 1080p: 1920x1080
• 720p: 1280x720
• 480p: 854x480
• 360p: 640x360
• 240p: 426x240