Is the image your trying to post have png suffix on the end of name or some other incompatible suffix? If so save the image as jpg or jpeg, in a suitable image viewer or editor app. I have Linux and use pix for that. In Windows Paint might work. I'm not certain though. When saving as jpg I delete the png suffix and replace with jpg. You should be able to upload the image then.
About the contrail length, I see that it is too short and also too dark. I did a bug report about it being too dark, during the beta stages of X-Plane 11. More people need to do this to raise its importance I think.