I am guessing it is one of two problems....
1) If you're trying to upload a photo in your photo album, you need to use the Upload a File doohickey below. You can also save images from the web by long pressing on them and selecting 'save' or 'add to album' or whatever they're calling it these days....and then posting it by using the Upload a File doohickey.
2) I am guessing this might be the problem....Safari now truncates URLs. What you see in the URL field is usually a truncated version of the actual URL, so when you're copying a pictures URL, you're getting something other than what you want.
So you have a three-step process...
First, long press on the photo and select Open in New Tab.
Second, once that happens, long press on the URL field, choose Select All and then Copy.
Third, come back here, click on the Photo icon above, and then paste the URL in the appropriate field.
Give either one a go. PM me if you're still f---ed up.