If you're using New Visual Editor, there should be an "Insert" button at the top, near Cancel & Publish. When you click it, you'll see the Media option. You can upload your picture from there.
If you're using Rich Text/Source Editor, then there should be a column at the right. The "Photo" button is right there and huge, you can't miss it. In some cases that column might be hidden. Look for a small arrow pointing left just next to the scroll.
I'm guessing you use Visual Editor. That editor has a bit of problem editing certain parts of the page. That's why I find Source mode much more useful. To access the source mode, you need to click the down arrow next to Edit button, and choose "Classic editor".
Pages including complex codes switch to source mode directly, but sometimes you need to switch to that mode yourself, using the buttons at the top where it says "Source/Visual".