Tumblr Dashboard Image Display Sizes (Updated March 20, 2016):
Photo Post: 540 by 810 pixels for dashboard view. Use 1280 by 1920 pixels for high-res version (except for superwide panoramas).
“Tall” Photo Post: Tumblr takes a 300-by-810-pixel version of your image then stretches it by 80% using HTML height and width attributes to make it 540 by 1458 pixels. Image quality may be diminished. Aim for uploading at least 710-by-1920-pixel images in case Tumblr switches to a better image size on the Dashboard. (It’s happenedbefore.)
Photoset: 540-pixel width for one image in a photoset row. 268-pixel width for two images in a photoset row. For three images in a photoset row, Tumblr displays 177 pixels on the left and right images and 178 pixels for the middle. Gutters are 4 pixels.