Images not displaying in downloaded themes

I am a newbie to WP as well as all things internet, so please bare with me. I thank you in advance for your patience.

After getting WP installed through godaddy (with help) in a directory, I found some good themes that could work for my portfolio and downloaded them to my computer HD in an organized folder called WP. (I am on a mac) I clicked the theme and activated it, but unfortunately, the boxes for the picts or the picts weren’t there! Just the header and the footer with a blank space in between (it’s a grid portfolio theme) And this with all the themes I downloaded, (installed as .zips). The only theme that displays correctly is the one that came with the installation of WP. Where are the images kept? or the boxes for them? Are there extra steps at installing the theme that I missed? in the info about the theme they write: “All of this theme’s files are located in /themes/gridportfolio”. where are these folders? on the internet?

Please be specific with any directions you may have for me, specifying step by step instructions. (I don’t understand what people are talking about in half the posts that i have read, searching for answers….)

If anyone has a good tutorial I can watch about this I also appreciate it. The ones I watched didn’t address my problem.

I really appreciate any input. Thank you.