On Tumblr on the computer (might not work in Tumblr mobile?) if you're writing a text post, you can write a bit of text, then hit enter, and you'll get a little popup with options (it's how you make cuts?) One of the options is a little camera icon, which allows you to upload a photo. Then you can keep typing.

(Ahahah I actually don't post on lj/dw like at all these days -- I don't even know if I've posted that I am on dreamwidth? keep meaning to, but...it's ironic, I love lj comments, hence me stalking your journal...but posting is hard.)

(My tumblrs are xparrot (http://xparrot.tumblr.com/) for general stuff, while idontneedasymbol (http://idontneedasymbol.tumblr.com/) is my SPN sideblog (as most of my friends on my main tumblr are not SPN fans, some pretty vehemently so, and I post a lot, so....)