I use this site on my iPhone and I'm not completely sure what it's like on computer If that's what you're using. But on iPhone I'd press upload image, then I'd have to optimize it so it would be under 300kb, then save the new image and select the optimized image instead of the original one then finally click the post button.
I know they recently changed the photo optimizer to be manual instead of automatic and ever since then its been an it difficult for the picture to turn out well.