In ImageMagick from PHP exec()
exec("convert -density XX image.pdf -resize YY% preview.png")
where XX is the desired density (nominal is 72) but I use something like 288=4*72) to get good quality and YY is the corresponding percent to reduce the image, such as 25%. The  means to process only the first page of the pdf.
exec("convert -density 288 image.pdf -resize 25% preview.png")
Adjust these arguments to get the desired output size. You can change png to jpg, but you should then add -quality ZZ before the output to set the jpg compression quality. Values are 0 to 100, nominal for ImageMagick is to use 92.