Topic: having trouble with tumblr

so how does one go about downloading/uploading FULL size tumblr images. I know already that you have to change the numbers to "raw" and all that stuff, and that's worked out pretty well so far. but now i'm having a different problem. how do you change a jpeg image into a png image? I've been trying different things forever now and I just keep getting errors so if anyone could help, that'd be awesome.

Updated by Mairo