Anyone know how to control the display size of an image?

Thanks for your input everyone, however I don't think I really asked the right question. Plus I don't think I've made a particularly good first impression.

I know that JPEG's are 'lossy' and I know how to achieve the best results when creating them. But what I'm having difficulty with is the difference in quality between the JPEG that I've created and how it finally appears on the forum. The forum software is resizing the image to fit the space. Even when it's already small enough to comfortably fit that space, it makes it bigger so that it fills all of it, and in doing so it is making it look considerably worse.

But if I put my obvious OCD aside, I'm sure I'll eventually come to terms with it

 

I tend to think in most cases just putting up a picture of some kind is OK for this "gardening" forum
Maybe if it was a photography forum you would need to go the extra mile, but mostly a good clear picture is good enough.