Ranchero's Big Cat Scripts plugin lets you run Applescripts and shell scripts from a contextual (right-click) menu on a selected file or folder. Scripts are stored in ~/Library/Application Support/Big Cat Scripts/Files. Here's a script to convert any image file to PNG:
on main(filelist)
repeat with this_file in filelist
set the target_path to (choose file name default name "newimage.png") as string
try
tell application "Image Events"
launch
-- open the image file
set this_image to open this_file
-- save in new file
save this_image as PNG in file target_path with icon
-- purge the open image data
close this_image
end tell
on error error_message
display dialog error_message
end try
end repeat
end main