Wow, what a lot to unpack! Rather than try to cover it all, let me offer my own somewhat similar quest as a guide (good or bad!).
First, you say you are "limited in computer skills", but does that mean you're looking to turn the whole project over to someone, or are you willing to select and perhaps selectively edit stock on-line images to come up with the representation you want to create for your layout. I'm sure there are those who, for a suitable fee, can guide you through the design and construction process for a background to meet your needs, but I'll assume you're a do it yourself'er like most of us who is willing to participate in as much of the process as possible.
With that said, you first need to determine what kind of image you want as a background for your layout. Prototype photos of the kind and location of the railroad you're trying to create or model would be a good start, as would a clear vision of the railroad you have built or are planning for the foreground. Once you have at least a tentative background in mind, you can start the search. For my layout, I had decided to add a tunnel covering a couple of existing tracks along the back edge of the layout, topped by rocky outcroppings at each end above the portals, so I was in search of a mountain view with a dip in the middle that would be compatible with and extend the layout's tunnel configuration off into the distance, with a reasonably dramatic sky above. After a suitable key word search, I found an on-line source for backgrounds (appropriately named Buy-Backdrop.com) that had a pretty good library of stock photos, among them the one below:
As part of my design process, I ended up cropping (to cut off the foreground and fit the height-width dimensions), blowing up, printing out (on about 40 sheets of paper, as I recall!) and taping together a mock-up of the background, and temporarily hanging it behind my layout (note: at this point, I had not yet started construction on the tunnel, which is represented here only by the portal propped up on the left):
Once I was satisfied with the effect, I ordered the cropped photo printed on a four foot by nine foot polyester fabric (vinyl was also available for a few dollars more). Since the supplier was overseas, it took several weeks to fabricate and ship it here (and this was pre-COVID supply chain problems!), but the cost was only about $20 total, so I was willing to wait. Local suppliers and chains like Office Depot offer similar services much quicker, but typically at *much* higher costs. Here's the final result, while it was still in the process of being fitted:
In my case, I decided to just hang the fabric in front of the wall (notched to clear a beam near the center) from hooks screwed into the ceiling, with a plastic pipe fastened along the bottom to both weigh it down and smooth the wrinkles, and serve to roll it up on to gain access to the wall and storage for rolling stock I'd previously installed there. In the alternative, the same type of fabric (or vinyl) backdrop could be taped to or permanently installed like wallpaper directly on the wall (if it's smooth enough) or mounted on some form of thin stiff backing that can be affixed to or hung in front of the wall.
Now as it happened, I was lucky enough to reasonably quickly find a stock background picture I was satisfied with, one that met my needs with only a few minor tweaks. If your needs are more complex (or if you're a perfectionist!), tools exist to create a custom collage of images that will match your vision for your planned layout, but you'll either need to develop those "limited computer skills" or find a friend who can do the image manipulation needed to come up with an image or images meeting your needs. The same supplier who fabricated my background (and, I'm sure, any local alternative) would be happy to print up your own custom image(s), in whatever size you desire. Depending on your layout configuration, you can either have the backdrop printed in sections, or you may need to plan on dismembering the print into more manageable sections for installation.
The one additional caution I'd offer is to be sure the graphic you have printed is as high resolution as you can create, especially if you are going to spread that image over a lot of real estate! Most stock photos offered by sites like the one I used are optimized to be printed large, but if you're 'rolling your own', just be sure to *start* with high-resolution photos or graphics. What may look perfectly adequate on a computer screen or printed on a sheet of paper may turn blocky and pixillated when blown up yards wide and hung behind your layout! You have been warned . . .
In any event, good luck!