This guide will teach you how to add a render image, a fixed image that moves with your screen, to your theme. You would most likely use a .png file of your photo cut out against a transparent background, which may require some Photoshop skills, but I can also give you some tips to avoid that.
To get started, we’re going to have to delve into your theme. So navigate to Customize and then into Edit HTML of your Custom Theme. If you scroll down, towards the top of your theme coding there should be a whole bunch of <meta name= codes, the ones we want are the ones with =image:. At the beginning, in between or at the end of those codes, slide in Code 1, as shown below;
Now what we need to do is scroll down until we find the coding </head>, it shouldn't be too far down from the meta name tags. Once you've successfully located it, place Code 2 after it, like so;
And that's all the coding there is to it! Now if you Update Preview and Save your theme, when you refresh the Customize page, there should be an option to add a render image much the same as a background or sidebar image.
Now, as for the image you want to use as your render, you're going to need to size it according to the height in Code 2, as you can see, right now it's 350px. You can change this to make the render larger or smaller, but take into account that it will stretch the image and may take away from the quality.
The renders that look the best are images of people, most likely standing, cut out against a transparent background. This looks quite effective if your posts cross under it. If you're not skilled with Photoshop, I would suggest you either; request one from a rpcha or use my little trick. I purposely went out and found a photo of Phoebe against a white background, then I look it to PS, edited the lighting and made sure that all the area around her was completely #ffffff. This way, I could leave a small lining of white around her and not have to deal with tricky cutting and shaping. Then, when placed against my white background, it looks completely natural. To show you, this is the photo against a coloured background;
It doesn't have to be perfect, because you're not going to see the seams anyway. Be sure to leave some transparent space on either side of your image, so that it's not right up against the side of your screen.
I hope this helped and I hope you enjoy your gorgeous render images!