Can you view the image on your phone? If so just screen shot it then trim the pic then put it on Instagram.

I export my edited video (from iMovie) to my desktop then send it to my phone. Then I use an app on my iPhone called "square ready for video" and convert the video sort it still stays "widescreen" view like an actual movie. Then upload it straight to insta. The only thing i can think of is perhaps the file is to large? And make sure it goes for 15 seconds or less. Otherwise it won't upload and you will have to cut either the beginning or end out. Hope this helps..

I know when I try and upload a video to Instagram, I've got to e-mail it to myself so that I can open it on my iPhone to then text it to myself so I can save it to my photo album (if I just upload it to my iPhone it goes into my movies folder and I can't upload it through insta as it doesn't access that folder). After that I then find it by going into insta and then clicking on the video recorder and uploading it. I think the video also has to be a .MP4.