my jpg stills distorted during compression on FCP

I shot a film with a 5dm2 (24fps) and stills with the same camera. I have made a film in FCP and have included many stills in the film. They look great in the viewer. Some of them are keyframed as well. I compress the video, according to all Vimeo suggested settings for FCP. ALL of the stills, in the compressed film come out terribly distorted and elongated. I went back and checked the attributes on the stills and saw that FCP had added, without my knowledge the "distort" filter. So, then, I went through every still, and removed the "distort" filter, recompressed and the same thing happened... all stills are distorted after working 2 days on trying to fix it. I am willing to redo ALL of the stills, if someone would just give me the correct, direct information on exact size, dpi, pixel ratio, and format. I have been trying to fix this for weeks to no avail. There must be solid information on what to do with my stills in Photoshop before importing them to FCP - but I cannot find it anywhere.
Information I need:
save as: format?
pixel dimension?
document size?
scale styles (check box?)
constrain proportion (check box?)
resample image (check box?)
PLUS: any advice on keyframing stills?

Also, my film in FCP is widescreen rectangle, and when compressed becomes a square, which also may be contributing to the problem... Please help! I would really appreciate it. Thank you in advance for sharing your wisdom.

Best,
Annie