Looks like you are posting the 'Thumbnail for Web' links. If you click through on them, you get a larger picture.
I edit my screenies with IrfanView, and resample/ resize to 1290wide jpg. When I upload to PostImage I say "Do not Resize" and then copy/paste the 'Direct Link' code. Use the pull down menu arrows to get what you want.
Add the img and /img tags like this, but use square brackets [], not curly ones{}
{img}https://i.postimg.cc/HsmgJGN9/Hawk-001.jpg{/img}
Hope this helps
