Make sure your image quality is high. Check your camera and take your photos so they are over 800 pixels wide.
Find out the size of your Blog Post. We need to know the main wrapper width. This tells us our maximum image width too. Click here to find out how to do that.
Next upload your image to Flickr. It will upload at the large size - and also offer some smaller sizes too. How useful! To see these varying sizes, you need to click on ALL SIZES above your image.
This will take you to a page which looks something like this::

See all the different sized images we can upload along the top of the image? Choose the one the same size as your blog post (main wrapper) width or a bit larger (because we can drag it smaller on Blogger later!). Click on that size. When the page has loaded, scroll down and find the box of HTML code UNDER the image (not the url address : we need the actual code in the box). Cut and paste that code into your new blog post (in COMPOSE mode). It'll be showing up as code still. Click PREVIEW. You'll see your image nice and big in PREVIEW. Hide the Preview and go back to your COMPOSE screen. Good job. We're on our way! Click Edit HTML. Then click COMPOSE. I know. That's weird. But we kind have to toggle a bit to get Blogger to show us our image!
Click on your image (which is now showing up in the COMPOSE window, rather than just weird old code!) and drag the corners to resize it a bit smaller if needed. Blogger will show you the width in pixels as you drag. (edit : just noticed that you can't drag the images to resize when using Google Chrome. you can when using Firefox, though.) Keep your eye on that and set it to neatly fit within your blog post column.
It might not be easy to get a full view of your image in the COMPOSE window, but never fear, it will show up when you PREVIEW and PUBLISH!
That's it! Hope this helps you to have lovelier images!
xx Pip