Use any image hosting provider such as imgur or epvpimg and post it between the img tags:
[img][/img]
Just make sure your link ends with the extension (png, jpg, .gif etc.) and as for your error, you may want to give more info about what were you wallet were you using, the OS etc.
It's a screen shot from my Android cellphone and I tried using Imgur to upload it on here. I'm not sure if it's a png or not