Heylinder, Your camera shoots much larger than that. Have looked at the specs for your camera you should easily be able to shoot a picture that is set for a pixel count of 2,592 x 1,944 then you could get a picture with 300ppi for printing at 8.6 x 6.5 inches. You will have to check your camera's instruction book to get the exact setting needed to come up with this pixel count. That ppi is actually referring to the print resolution and 8.6 x 6.5 inches is the largest photo you could print with that pixel count on your settings without losing quality.
I believe what Staples wants is for people NOT to compress the file they are uploading, because if you compress the file then the picture will lose quality.
I can't find anywhere on their website stating how large "inch wise" the photo must be...
Really the only camera which can't come up with 300ppi is a 1 megapixel camera all the other megapixels can produce a picture capable of 300ppi but the size of the printed photo to maintain that 300ppi will vary. For instance, a 2mp camera can produce a 300ppi picture but for that to be maintained the print size can be no larger than 4.0 x 5.3 inches. While an 8 mp camera can produce a 300ppi print that can be as large as nearly 8.5 x 11 inches.