Why Won't My Png Files Saved In Corel Paintshop Pro X9 Ultimate Upload Properly?

Having a problem understanding why PNG files I was posting a couple of days ago would upload ok, (using multiple images upload - all five used) but after going through writing descriptions and creating tags then "submitting" them when finished working on all five, they wouldn't load to the site. Wasted a couple of hours worth of work (or more) on this process. All five images were each less than the 25MB limit, being 13-16MB each. Should have been fine to load them. I even tried one at a time after the first attempt; still wouldn't do it. Later that night, I tried again; still wouldn't work even one at a time. The only thing I could think was different about these images is that I'd worked in a new program (for me), Corel PaintShop Pro X9 Ultimate and saved as png files in that program, instead of working in Photoshop Elements that I normally use. Out of curiosity, I put the png images in PSE and re-saved as png files then tried again, one at a time. This time I gave the image only a name with the uploaded image and it did submit. Then went back and did the descriptions, etc. afterwards. So, my question is, why would it upload the PNG image saved in PSE but not in the Corel program? Can anyone offer an explanation? It seems odd to have to work in one program and save the image there, then have to open the other one and re-save it in the same format! Doesn't make sense to me.