I find with the image optimizer you have crop with width and height, and can only do so much but then there is a 2nd option after the crop. This option has width and sharpness. Change the width of the picture there and it will come down below 300kb and be able to post. It basically shrinks the picture.
Took me forever to figure that out. If I left it as my phone does it I'd be well over 600kb and more like 1000kb and getting nowhere.
I was using a 3rd party program to make the pictures smaller but it's not necessary usin the optimizer tool like that.