Computer screens don’t use anything above 72 DPI, so having a file bigger just causes download problems without any extra benefit to the screen picture.
Blogger / Google give you an allowance for photographs, as I understand it, anything at 750 pixels and above in one dimension come off the allowance, while anything smaller is hosted free. Normally a full size photo for the screen would typically be 800 x 600 pixels, but if the above is true, then slightly smaller is better.
There is a big difference in file size between an 800 x 600 image and say a 600 x 400 image. There comes a point in which you can compress the file size so much that you end up with a grainy picture.
I have come to the conclusion that it is better to have fewer photo’s, but go for an image that is 600 – 700 pixels wide.
For my iPad I use an image size converter called Image Size. I can manage my blog on the iPad with the Blog Touch Pro app
I prefer to handle my images by the resize function that comes built into the photo editor on my Chrome Book.