As far as I know, you can pretty much insert html anywhere in a PHP document, and it will be accepted. It’s a matter of where you want it to appear. Say you want to add an html img src code to every post. You’d go to the index.php, look for the post content listing and throw the html in there. If you want to do the same in the sidebar, go to the sidebar php and stick it where you want the image to show up. However, you never want to break up the php code. That’s bad. Stay from outside of the <—-> sections, and you should be okay. As a coding idiot, what I did when I first started learning placement was to save the file that works as goodheader.php (or goodsidebar.php, etc) in your files, then mess around with placement in the existing file, and keep seeing how it looks. Worst case scenario, you can always change the name good—.php back to the original name and nothing’s changed. But that’s just my two cents. For an example, look at my site: monkeypup.org. In the sidebar, I have images in place of category headers. What I did was put an image tag in the sidebar.php in place of the category header. So…does any of what I said make sense? If not, there are far more intelligent and capable people here to help. Might also help to know where on your page you want the html code to appear…
Good luck!