I tried uploading a png to the image host you're using and it went through fine. The only way I got a jpg was if I uploaded a bmp. I imagine this is what you are doing. If you aren't sure what kind of file you've been uploading, then try this the next time you're on tiny image's website. Take a look at the file address that shows up after selecting your image with the browse button. Your filetype will be shown at hte very end of the file address.
EDIT AFTER READING PREVIOUS POST: I don't use tiny pic very often though, so perhaps my experience isn't typical. I imagine they probably just check the file size and if it exceeds some limit for a certain size then they convert it to jpg. I tried uploading an 800x450 png that was 140kb and it uploaded fine as well. I still think you're uploading bmps.
