It’s most probably the resizing element because internet based services aren’t as good as resizing without compressing, I believe. Do you have any photo editing software on your computer? It would be best to use that to resize. If not, then perhaps try a simple free image resizer, I have “VSO Image Resizer”, which I use frequently without the compromise for quality- perhaps give it a go. It’s free for personal use but you have to obtain a license for it to be used commercially, good luck and I look forward to seeing some more photographs from you soon!