I don't know about Image Share, but I've never been able to get tumblr working on my 3DS. Whenever I try to log in, it just brings me back to the same page (or the sign up page, I don't remember too well.) I've always just assumed that something in the 3DS's browser, such as a flash component, java, etc. wasn't up to date, and as a result, wasn't able to use tumblr.
Usually popular sites like facebook, Google, and Yahoo! have mobile forms of their sites, but I'm not sure that tumblr does. If it does, it must not recognize the 3DS as a mobile device.