Imagemagick is a swiss-army knife of command-line image conversion, but can be a bit complicated to actually use. I have been making most of my figures with R lately, and printing them to pdfs, which I can include very easily into documents with pdflatex. I like pdf because it is scalable, fairly small file size (smaller than .eps), and portable. But today a colleague wanted to include a few of my figures in her own powerpoint presentation, and powerpoint only likes bitmaps. She was just going to take screenshots of the figures, but I quickly said, “no, I will just convert them to pngs”. She replied: “I don’t want you to go to a bunch of trouble.” “No trouble at all,” I replied. Then I quickly wrote a bash for loop to convert all the pdf figures into pngs. Then an hour later when I went to zip them up and e-mail them to her, I realized that they looked like crap. After a bit of searching online, I found the flags I was looking for, and eventually used:

for file in *.pdf; do \
echo $file;\
convert -density 600x600 -resize 800x560 -quality 90 $file `echo $file|cut -f1 -d'.'`.png;\
done

And now the code explained:
-density 600×600 says treat the pdf as 600×600 dpi resolution
-quality 90 says use the highest compression level for png (9) and no filtering (0)
-resize 800×560 gives the dimensions in pixels of the resulting png file

Happy ImageMagicking!