The first one must be some kind of temporary blip because an upload should either be a success or a failure. My guess is that if you were to try and refresh a few secs/mins later then it would probably show up.
As for the error message one, yes - it is probably too big - particularly if you already know it is a high-res file.
Images are automatically downscaled if they are beyond a certain size but also images are rejected if they are too big.
I don't know if a more meaningful message could be displayed in such instances as that one is quite generic.
Anyway, those who post full-width of screen hi-res images are the ones using a different image host. FreeOnes is a a reliable place to upload attachments but it comes with the caveat that they come with a cap when it comes to size and quality.
Years ago, I bought FotoFinish for .. $100? It is a decent digital photo editor. AND, one of the options is to Resize a picture. I use it often, to post stuff on FO. (I usually go no larger than .. 600KB.)