Due to blogger issues, the pictures are displayed at 1600 px by default (s1600 parameter in the url). To now the best image resolution available in blogspot is 2560 px. All the images of the blog will be uploaded in this section at the original quality.
Update: Now Blogger seems to recognize the original size of images. If you are not sure you are viewing the real size of the picture, rewrite in the url the parameter "s####" to "s0".