The Tumblr Image Post size (1280×1920px)
Tumblr's dashboard displays images in a single vertical column, and it favors tall, high-resolution photos, so 1280x1920 pixels at 2:3 is a great fit. Tumblr scales images to roughly 1280 pixels wide in the feed, so uploading at this width keeps your post crisp without triggering unnecessary downscaling.
Because Tumblr is a visual, reblog-driven platform, vertical images perform well, taking up more of the dashboard as people scroll and reblog. The 2:3 portrait ratio gives your artwork, photography, or graphics strong presence in the feed while staying fully visible rather than getting truncated the way extremely long images can be.
When your post gets reblogged, it travels across many blogs with different themes, so a self-contained image that reads on its own works best. Keep any important detail comfortably inside the frame rather than at the very edges, and upload at full 1280x1920 resolution so the image holds up when viewers click through or when high-density screens render it.