No, you don't have to donate (there's no donate facility for the forum) & you don't have to any client's Pro version.
This forum has nothing to do with LW apart from offering advice to LW users.

Don't really know why you're having problems, but try resizing the image & then saving it to your HDD. After that, upload it within User CP here.

That's what I've done... resized my image & saved it to HDD rather than hosting it. Then uploaded it.

I had probs for a short time on another forum, with an avatar which was hosted on Photobucket but I think in that case I was originally using the wrong link. Image > click on Share (above your image) > Get Link Code > click on the first option 'IMG for bulletin boards & forums'. Copy that link.