First, get a wider blog template. Simply go to Layout and click Pick New Template. Mine is Minima Stretch. Any kind of stretch will do. Just don't hurt yourself.
Second, choose your photo and resize in Photoshop or any photo program. I like 500 pixels width for vertical photos and 700 pixel width for horizontal shots.
Third, load them into Photobucket or a similar photosharing site (it's free, I'm not trying to break anyone's budget here).
Last, when you are ready to put picture in your post, switch from Compose to the Edit Html window. In Photobucket, click on HTML Code and it will copy it for you. Come back to your post and click control V. Back and forth, back and forth (I keep Photobucket and Blogger open) and in a minute or so you've got them loaded. Truly, it's faster than waiting for the blogger upload.
Now you can get ickitypickitty and head back to Compose to center your photos and write cute little captions under them. Lovely.
Examples? Twist my arm....