I hardly know what I'm doing...but what I did worked, so here it is.
first you'll have to make your blog big enough to handle bigger pictures
1. in your html find the section titled Outer Wrapper
2. change 3 values: outer wrapper (the width of your whole blog), main wrapper (where you post), sidebar wrapper (sidebar....if you only have one that is)
these can be anything but keep in mind that other computer screens may be different than yours and that the main wrapper and sidebar wrapper need to fit inside the outer wrapper
*fyi to give you an idea: MY outer is 820, main is 510 and sidebar is 220 (that's why there's the 90 pixel or so gap in between)
3. get it how you like it and save it. you won't have to repeat this part ever
then you just have to start posting huge pictures (with a couple easy changes)
4. upload a picture (just to blogger) before I do this I change the dpi from 300 to 72 in photoshop. this just saves space and makes uploading quicker. that high of quality won't show online...save it for printing. it depends how they come from your camera though.
here's one from 2006
5. edit the photo's html by clicking that tab next to compose
you will be changing the width and height. on my blog I want the width to always be 500. since this one is a square, height is also 500. then search through all the garbage slashes and AAAAAA's and find the s400. or s240. whatever s number you have (depends on if you chose small, medium or large upload). try doubling it. this is what bumps the quality back up. s800 has been working for me though one site said just to do s1600. I can't see the difference. I don't think it matters.
*algebra time! so you'll need to figure out what you usually need to change your pictures to. a picture out of my camera at 300 dpi is 2592 pixels by 3872. when I change the dpi to 72 for web viewing the dimensions become 622 by 929. (still big enough to post big you see) when I do post it I know I want the 622 side to become 500. so 500 times 929 divided by 622 = about 747. I'm going to want to change my numbers to 500 width and 747 height. if I do the same thing for a horizontal picture it will be 500 width by 335 height. figure out what size comes out of your camera and write it down. you'll rarely have to use anything else. also if you use photoshop or something, in the image size dialog box you can enter in one of the dimensions and the other one just automatically changes. that's easy too.
mess up the proportions and you'll get something like this:
neglect the s number and this will happen (see the blur? not good):
time for a little more practice for me
p.s. sometimes when you change the html on a photo and click back over to compose it takes just a s e c o n d to pop up. wait for the little red x box to go away before you get yourself in a tizzy