Thanks! Uploading multiple images in a single post is currently not possible due to limitations in the Tumblr api and Web Extensions api. While the Tumblr api allows you to upload multiple images to the same post you have to send the actual binary data for the images in an array to Tumblr and the web extension api doesn't have a way of getting the binary data of the images that you want to post.

Maybe someone else will make a Chrome extension that works for you. I don't care about Chrome at all since it's an inferior web browser.