If you find a template you like, is it possible to change the picture in the template to something closer to your theme? Thanks.
everything is changeable - just depends on the template and what you want changed -- be it the header/background/footer -- just check the code and adjust ....upload your own image etc..
Ever single thing is changeable, unless a linkback fro the author is required..other than that you may changed a theme as you wish, sometimes with suprisingly pleasant results!
But I mean like how? I have basic knowledge of HTML but I don't understand the PHP code and I don't want to comungulate anything. Basically like how do I add a nav bar and banners and basically just do whatever to edit without touching the PHP.
look in your theme directory and then your images directory. Figure out which picture you want to change and note the pixel dimensions of the image. Then make your own image of the same pixel dimensions. Give it the same exact name as the image you want to replace it with, and upload it to your theme image directory. This will overwrite the old image and now the wordpress theme will display your new image.