Looks like everyone’s having this problem. Here’s how you fix it:

Open your inbox in a new tab, keep tumblr dashboard or your post page open for this. Do not close it.

Open a new text (http://tumblr.com/new/text or http://tumblr.com/blog/yoururlhere/new/text) and upload the icon there. Copy and paste it into the ask, close the text (if you want to keep on the dashboard, just hit ‘cancel’), and there you go.

I don’t have pictures because I don’t have an ask right now to answer. But this is how I’ve been doing it.