Images uploading at low res

Hi I'm having problems with images not uploading at full res, I go through the normal process of clicking on the editable pic, choosing the jpeg, setting the size (as a 100% width no height to keep the aspect ratio correct) etc it then previews ok but when I publish the changes it uploads a tiny low res file to the server.

I've found that if I first drop the image in at it's original size with pixel dimensions it gets stretched to the full depth but if I then subsequently change it to a percentage width it will be fine.

There must be a better way than having to do everything twice!