You can use pdftoppm from the poppler-utils package to convert a PDF to a PNG:
pdftoppm input.pdf outputname -png
This will output each page in the PDF using the format outputname-01.png, with 01 being the index of the page.
Converting a single page or a range of pages of the PDF
pdftoppm input.pdf outputname -png -f {page} -singlefile
Change {page} to the page number. It's indexed at 1, so -f 1 would be the first page.
If you'd like to work on a range of pages, you can also specify a number for the flag -l (last page), so having -f 1 -l 30 would specify the pages from 1 to 30.
Note again that .png will be appended to outputname automatically, so there's no need to include the extension. Also, -singlefile removes the -01 suffix cited above, since the output is known to have only one file.
Specifying the converted image's resolution
The default resolution for this command is 150 DPI. Increasing it will result in both a larger file size and more detail.
To increase the resolution of the converted PDF, add the options -rx {resolution} and -ry {resolution}. For example:
pdftoppm input.pdf outputname -png -rx 300 -ry 300