(retitled, moved to gnome-utils screenshot)

Perhaps the dialog that appears when you hit Browse would be a suitable place to
allow users to select other formats supported by Gdkpixbuf.  

Screenshots saved as JPEGs (with any useful amount of compression) are
basically 100% guaranteed to have ugly visual artifacts, because screenshots
are about as close as you can get to a worst case for the JPEG compression
algorithm. Likewise, screenshots of modern desktops saved as GIF or BMP are
almost always going to blow out their colormaps and have ugly pixelation. And
IE6 can't display TIFFs.

PNG is almost always the best choice for a screenshot. So if we're going
to add this functionality, something like Alan's suggestion (which hides it
sort of out of the way) is better than the screenshot mentioned on d-d-l
(http://log.emmanuelebassi.net/images/shots/screenshot-expanded-format.png),
which presents it very prominently in a way that suggests that the user needs
to actually think about what format he wants to use each time he takes a
screenshot.

the artifacts are acceptable if I want to screenshot the whole desktop: a 1024x768x24 bit PNG (my desktop) is 1.1 MB - pretty much unacceptable for people with low bandwidth or if I want to share my desktop on a location offering bandwidth and/or size constraints.

while the UI suggestion is an half-assed attempt done in 5 minutes with glade, the settings are (or should always be) saved; thus, the user doesn't need to think about the format unless he/she *does* want to think about it.  the whole point of transferring controls on the screenshot dialog is to avoid having to open another dialog.

also, having the format chooser inside the file selection dialog would make it unaccessible when saving by drag and drop.

*** This bug has been marked as a duplicate of 325187 ***