>>214033371
Usually the cause of this is that whatever image you've downloaded from elsewhere with the intention of posting on 4chan has its permissions Locked.

Happens often when you're stealing an image from a proper website like a newspaper or some such. If you open it in whatever image viewer your OS has and then export a new copy, the new copy will be unlocked and it will let you upload it to 4chan.

It's a pain in the arse though, I notice it happening more and more recently. I should be able to steal and use whatever images I want.