It would help to know what type of software you are using.
if you have elements its very easy.
just go to the top menu bar beside file and edit. click on image, then image sze. a box will open. make sure your constarine proportions and resample image boxes are checked.
go to the resolution box and put in 72 (also the box next to it should be set to pixels per inch and NOT centimeters.)
see how you get on with this. but most software should work along the same lines.
worst case scenario is there is somthing wrong with the image. a picture with the dimentions you mentioned should, if a JPEG be about 2-3Mb if not then that may be the problem.
hope this helps