To post pictures in text post you need to hit the <html> button, input this code:
<img src=” ” />
and between the quotation marks put the url to the image.
<img src=”http://media.tumblr.com/5d97a05f33bd4c60aae1cedb8adc1b99/tumblr_inline_mh3ir2VCXP1qimtha.gif” />
Idk I saw a couple posts wondering how to do this and thought this might be helpful for some :)
Otherwise you can just hit the camera icon in the shortcut bar and upload a new gif.