Full photo not appearing at max size

hello, again, going spare about this. I have a very simple drawing blog where all I do is post drawings inserting the image at full size in the post and adding a couple of lines of text underneath. I prepare the jpeg at 800px wide which is the max size the blog allows. This has always been fine, only upon visiting recently I noticed it’s not working. I have had a very thorough look through the CSS stylesheet and there seems to be no problem there. I have also made sure that the setting are correct in the Media settings, and it’s all as should be. Why should this not work? oh, help. Thanks so much.