How do you put your picture on your page on Tumblr?

You can either get a theme that allows you to have a sidebar image (profile picture). Or you can upload your picture to Photobucket and get the photo html code and paste it into your "Description", To get to your Description box, you just go to "Customize appearance".

I think what you mean is that you want a sidebar picture. Most themes allow you to simply upload one under the appearance tab in customize but if not, you can just insert it in yourself. All you need to do is find the picture you want and upload it to tinypic.com. Then copy the direct link code of the photo it gives you. Or if the picture is on facebook right click the picture and then click copy image url/image location. Now, put this code in your description: <img src="IMAGE LINK HERE"/> and you would replace image link here with the image url you copied. Also if you want to change the width and height so the picture will fit in your sidebar, you just add this: <img src="IMAGE LINK HERE" width="#" height="#"/> then replace the #s with numbers and play around with different sizes until you get the correct fit. good luck!