Folder Action is what you want;
pdf2jpg.scpt
on adding folder items to this_folder after receiving these_items
repeat with an_item in these_items
tell application "Finder"
set file_name to name of an_item
set file_ext to name extension of an_item
set desk_path to (this_folder as string)
set pdf_size to (size of file an_item as integer)
repeat until pdf_size > 0
delay 1
set pdf_size to (size of file an_item as integer)
end repeat
if file_name starts with "Picture" and file_ext is "PDF" then
set format_return to display dialog "Select output format." buttons {".pct", ".jpg"} default button ".jpg"
set output_format to button returned of format_return
set AppleScript's text item delimiters to "."
set base_filename to the first text item of file_name
set img_path to desk_path & base_filename & output_format
tell me to convert(an_item, img_path, output_format)
end if
end tell
end repeat
end adding folder items to
on convert(img_file, img_path, output_format)
tell application "Image Events"
launch
set image_ref to open img_file
if output_format is ".jpg" then
save image_ref in img_path as JPEG with icon
delete img_file
else if output_format is ".pct" then
save image_ref in img_path as PICT with icon
delete img_file
end if
close image_ref
end tell
end convert