for the 8x10s i used the HP premium high gloss or ultra gloss or whatever they called it and the best quality setting in the printer properties. for the 4x6s i use the kodak ultima high gloss paper and again the best quality on the printer.
i have never printed on this other than standard documents and photos, but the photo cartridge does do a fine job no doubt.
to print a bigger picture than you have so far you may need to manipulate the photo some. just about any photo you find on the web will be of standard monitor 72 resolution. about the best pics you will be able to print will be at 300 dpi resolution (despite the printer and all printers claiming something ridiculous like 4800 by 1200 resolution). i use adobe photoshop to change the resolution of pics i find online. i don't know of another program out there for free that can do it for you. photoshop is around $500 and honestly worth the money if you edit pics often. if you just print pics from the web without adjusting them they will be small as [censored]. if you blow up the resolution you will lose some detail but gain all the size you want. i have blown up the ozone's pics to 300 pixels per inch to get a near 8x10 size and 200 pixels per inch to get near 4x6 size. you can blow them up and they still look pretty good printed, especially when you put them in a frame and don't get within inches to look for blockiness in the pics. one note here, when you do blow up the pics they will look a little blocky on your monitor but 99 percent of that fades away when you go to print.
the best free image editor out there bar none is
www.irfanview.com . i don't believe it has settings for dpi when you blow up pics but i know you can enlarge the overall size for sure.
you may want to look around on
www.suprnova.org and see if you can find anything there to download that may help you. it is a bit torrent file source and they have lots of image editors there. here is the page for the free bit torrent program i use to download these (
http://azureus.sourceforge.net/ ).
if you have any questions let me know.