Arsalan,
I'm sorry, it did work as you stated.

I uploaded everything to the wrong site as I am multi tasking instead of focusing on one at a time.

I guess Forest Gump was right "Stupid is as Stupid Does"

Thanks for your guidance and patience much appreciated.

Fair enough.
Here is what I've done that has failed. Keep in mind my actual theme name isn't goblue but for clarity I am using goblue in my descriptions below.

1. I created a new background image the same size (1501x763) as goblue/images/background.jpg and renamed it background2.jpg.

2. Uploaded to the server and deleted the original background.jpg image.

3. Went into goblue/plugins/default/css/core/default.css and changed this style to reflect my newly uploaded background2.jpg image.

   .ossn-layout-startup-background {
   min-height: 560px;
   background: url("<?php echo ossn_add_cache_to_url(ossn_theme_url('images/background2.jpg'));?>") no-repeat;
   background-size: cover;
   }

4. Searched through every line of code within all the files inside the goblue theme folders looking for ossn_theme_url(images/background.jpg) and found no other paths to that image.

5. Went into mydomain/administrator and clicked the flush cache button inside the cache box near the bottom of the page.

6. Went back to the website and cleared browser cache and the image remained the same no changes at all.

Here are some screenshots to show the two different images and the flush cache button.
https://www.dropbox.com/scl/fi/fmj08j92cqcpgjm4wxgn7/Screenshot-2025-11-29-113944.png?rlkey=qmm692jl4zgm8ysxh5wrna31g&st=uhmegu17&dl=0
https://www.dropbox.com/scl/fi/sp89ku1s87mcbapbbbeaw/background2.jpg?rlkey=cucgxedl29hzyeojpdcxjr3mo&st=0x4i1olm&dl=0
https://www.dropbox.com/scl/fi/npa0ivtbk145bkgrj7td4/Screenshot-2025-11-29-112753.jpg?rlkey=0gx3vymd3iop2j4qbi1n7xc2o&st=snmn21x4&dl=0

Without looking the code i have no idea what is wrong.

You can simply use original theme.
Replace file you want background image in images folder
Flush ossn cache
Clear all your browser cache

Arsalan, unfortunately I have access to my site password protected until I can finish it so cannot provide a url to it right now.
Also I am not seeing any errors, the background image isn't showing my changes that is all.

If you mean changing the css for the image when you state "only change in actual file and clear osdn cache thats all" that is what I've done along with replacing the actual background.jpg image inside the theme images folder where the existing one I am trying to change exists. I have cleared the osdn cache on the admin dashboard. Is there another file(s) somewhere I cannot find that needs to be changed also?

If you just struggling to change image only change in actual file and clear osdn cache thats all

If you doing by creating own theme make sure you updates all paths.

I said before that its difficult to know what is error or where you are doing wrong without seeing your code. So you need to upload your theme somewhere