If the images came from your phone camera then they're likely over a megabyte in size.

I used to experience uploading issues until I started resizing my pics with the "Image Resize" app.

My camera's images are typically 1-3 mb. When resized they shrink to around 100 kb.

I've had no problems posting since resizing.

You can get it on Google play, F-Droid, or GitHub.

I'm sure that the iPhone store has a similar app.

I hope this helps