Mike,
I understand that it depends on how far I want to zoom in.
However, what is project resolution ?
I'm just 'editing' and 'throwing' a picture in the mix. How do I appreciate 'project resolution' ?
I've tried 665x310 (turned out almost perfect), 1600x1025 (not so good), 500x333 (not good at all), 786x464 (not perfect), 586x426 (no good), 2592x1944 (fair), 1024x768 (not so good). What range does it ? It seemjs like higher / lower does not always correspond appropreatly.
What am I missing ?
Is there a 'golden rule' ?
Thanks for your help.

How do you not know what your project resolution is? Go to File / Properties and you'll see three items of importance listed there: width, height, and pixel aspect ratio. Multiply the width by the pixel aspect ratio, then use that result and the height to determine the "ideal" still image size.

Alternatively, some folks prefer to use the listed width and then divide the height by the pixel aspect ratio. This often results in a slightly different size image but should give just as acceptable a result.

Personally, i feel the "more than twice project resolution" is a very poor rule. As you've discovered, images that are larger may not look as crisp after Vegas reduces them. It's good to decide what size the images need to be and resize them first in a photo editing program which will probably do a much better job than Vegas does. If you don't need to zoom in at all, then "twice project resolution" will give a poorer result than "same as project resolution". If you need to zoom in further than 2x then you need to calculate the size based on the zoom factor you will be using, rather than an amorphous "more than twice".

While 2x project rez is a good general rule.. the caveats are that your merge into Vegas timeline edits from still sources requires:

1) A good near lossless format like png type - NOT JPEG ever ever!
2) Color grading management will get tricky depending on what the source of the file is -you must consider aspect ratio, gamma, color encode, and real pixel rez of the source.
3) Dimensions of the "cropped' picture should be very similar to project aspect ratio - like 1440x1080 is going to work better than 1449x1100 .....

To be honest, I think you're over complicating things. I've done a lot of this, and it's only when you get a very small LOW res pic that it matters. If it's at a res suitable for printing (Say 180dpi and above) it'll be fine. Yes, lossless formats are better - but I've been given probably '000's of .jpgs by clients to use and they look OK. Be aware that Pan/crop is the best tool for this. Track motion loses more res when zooming in.

"If it's at a res suitable for printing (Say 180dpi and above)"

The dpi is irrelevant in video work. It only affects how big a picture will appear when it is printed.

In video work all you need consider are the absolute number of pixels. Work out how much you plan to zoom in and multiply the project resolution by this factor to get the minimum resolution of your stills. You should not use resolution much greater than that otherwise you risk stressing the memory requirements, leading to sluggish editing performance and even a possible crash.

A few large stills are OK but keep them under control for best performance.

A JPEG 'high' still (not even max) probably beats by a long shot any type of lossy video compression. There is no reason to fear good Jpegs.