I think I figured out the issue. It may be that the photos that you are uploading are too big for the KK forum to handle.
I don’t have any insight into how eBay works, but I’m sure that eBay has good software on their end that can accept a large picture and scale it down for displaying in an auction listing. The fact that you can upload photos to eBay, but not to the KK forum regardless of which computer you use indicates that it’s not a problem on your end.
I just did an experiment. I tried to upload a full resolution picture off of my camera that was 5.6 MB and 4896 x 3264 pixels big. I tried both the advanced uploader and the basic uploader, and neither one worked. I’m using a Mac. I then took my photo and downsized it to 1200 x 800 pixels, which brought it down to 0.46 MB (461 KB), and it uploaded without problems. You don’t say what kind of camera you have, but I bet the photos you have are at least 2-3 MB.
Try taking one of your photos, make a copy of it, and use your computer to downsize the resolution of the copy of your photo so that the photo is about 0.5 MB (500KB). Someone else will have to guide you through that process, since I don’t use Windows. But I’m sure there’s a utility program that came with your computer that will be able to do this. Then try to upload the smaller picture and see if that works.