JPG pics embedded in MP4 change display size/aspect ratio while playing

[FrustratedBiographer]

I have a weird problem that is driving me crazy. I can't seem to web search with the right terms to find a solution either. I am producing my own biography in an MP4 format. I have a lot of content such as pictures, short videos, etc. that I've put in my source editor and successfully produce an MP4 with. However, I've noticed that SOME of my pics resize themselves on screen during their 6 second play. Not all of them, just some. Videos don't do this, just the pictures. Any ideas or help is GREATLY appreciated. Here's what I've done so far:

1) I have tried resizing my pictures to different resolutions and also resized the video resolution, i.e. forcefully changed to 1920x1080, or 1280x720.
2) Changed FPS up and down, i.e. from 25 to 30
3) Changed editors - while I don't use high-end product I have used Filmora and Movavi as well as Openshot. The problem with SOME photos follows the end product produced.
4) I edited the dpi in the photos in question from 96 down to 72

I'm out of ideas, but my OCD is not allowing my mind to accept that some of these photos "tweak" themselves during my movie, and so I thought I would throw this out to the metaverse and see if anyone has any other ideas or can point out my stupidity in a gentle way.

Thank you.

[ProWo]

The pics must have the same resolution as the video, g.e. 1920x1080.
Vertical pics must be padded to obtain this horrizontal resolution.

[FrustratedBiographer]

Thank you... Yes I did consider that possibility. Here's an example of a problem child pic though....

Original photo was scanned from a printed picture. It resulted in an electronic file with:
Resolution of 1739x1478, 600 dpi

I wanted the same aspect ratio only smaller to fit the video resolution so I edited the photo and it's now:
923x720, 72 dpi

So yes, this would be below the resolution of either video format, and the photo basically crops itself vertically on both ends during playback... i.e. it looks like it initially shows at that resolution and then self adjusts to maybe 820x720 for the last second or so. It's just weird. I have other source photos from my iphone at 4K implanted that don't do that. I assumed it was just a bug in my video editor for a long while.

In any case it might be worth a shot to try. Do you know of any utility programs that would "pad" my source to create an output matching the video resolution?

Thanks again!

[jagabo]

As ProWo said, you want to add borders (230 on the left, 230 on the right) to your 820x720 picture to make it 1280x720 (assuming the rest of your video is 1280x720). Most players don't adapt properly to different frame sizes within a single MP4 file.

[FrustratedBiographer]

Ok thank you both for that input! Much appreciated.