First I edit my photos on Picasa, one of the best tools ever. It's free and I highly recommend it.
Next, I go to File (uppermost left corner) and choose the option "Export picture to folder." It's at that point I can choose what size to make the pic. I almost always go with the 640 pixel option, but I go bigger with complicated pics I want to really emphasize. Pay attention to the location of the folder that you are sending the pics to.
HINT: if you have many photos, name them the same and follow with a number, such as: Khorsabad 1, Khorsabad 2, Khorsabad 3 - it will make uploading them in a specific order much easier.
Create a Blogger post. On the row that has the font options, go almost over all the way to the right side. See the pretty picture of a mountain scene? Click it and you'll get the Blogger Upload Image box. In the upper left corner it says "Add an Image From Your Computer" - choose the browse option and go find the folder where you exported your pics to from Picasa.
Doubleclick on the name of the photo you want and choose your layout. I always put my photos in the Center, that way I don't get text doing weird wraparound things. For image size I generally leave it on Large. Click Upload Image and it will get sent to your blog entry.
Now this Upload Image function is quirky. It works in reverse order. So for complicated posts I write down the list of the pics I want in the proper order. And then I load them starting from the bottom of the list.
Load photos one of a time in this reverse order and you can load as many pics as you want.
Not to get confusing, but there is a quicker option. After you've selected one photo, it has the option of adding another photo. I don't recommend doing more than four at a time because the capacity is limited. These photos will go in the order that you select. BUT if you then choose to do another batch, they will go on top of this one, so you really have to pay attention to what you are doing if you want them in a certain order.
Also check out Blogger's photo help.
I hope all of this didn't confuse you further. Ask questions and I will try to clarify. There are plenty of other ways to upload photos, but this is how I do it.