Last year, we made changes to our Community Guidelines which may have impacted how your header or avatar is displayed on Tumblr.
If your blog was self-flagged or flagged by us as explicit per our old policy (before December 17, 2018), your avatar and header have been reset to default. If you try to change your avatar or header while your blog is flagged as explicit, the changes will not be saved. You’ll need to appeal your blog’s explicit classification before you’ll be able to make any changes.
You can check and see if your blog is marked as explicit in your visibility settings.