Changing Photo Type

[Lov3shy]

I use online picture modification sites (such as Picnik and Fotoflexer) to edit pictures both for fun and as an aid to drawing better (copying the outline without having the rendering annoy you).
But my only problem is that (with Fotoflexer), the picture type is unrecognisable and I'm unable to open it in a document. I use Fotoflexer more because it's better for things like concealing spots or other unsightly areas; Picnik are not as efficient in this.
I choose the program to open it in (Windows Picture & fax Viewer) but it doesnt stay permanently in that program.
Is their a way of changing the photo type semi-permanently (To which you can change it to another type)?
Also, do you know any other picture modification sites (they have to be online, not downloadable) Thanks