To understand the who pdf/x thing here is an excellent reference. I send this link to clients all the time. http://www.globalgraphics.com/technology/pdfx/
....jpg large logo, 300dpi, png large logo 72dpi.....
You would do well to learn more about dpi. So many people do not understand how dpi / resolution works. A 100px x 100px image at 72dpi is exactly the same as 100 x 100px at 300dpi. In reality what is most important is pixel dimensions. In fact there is not such a thing as a 72 dpi PNG. PNGs do not preserve any dpi data.
Here are a couple of good links about the "DPI Myth"
When I am designing graphics for the web where it is the client that will be putting them on the site, I usually tell them, "Please give me the actual pixel dimensions you need, and I will create them for you." Why? Because too many times I provide a high res raster image, and they either put it on the site directly, and just use the html to scale it, or they resize it in a image editor, and it comes out looking awful.
If they say they are using Photoshop, I often tell them I will send them a pdf file that can be opened in Photoshop. Then I just provide them with a pdf/x with the layers named, so that layers are preserved in photoshop if necessary. (like if the graphic has a shadow layer). Why? Because when they import the pdf into photoshop, they have to choose the dimensions that it will be rasterized at. Instruct them to open it at the exact dimensions they need for their web graphics. After it is imported in this way it is good to tell them usually sharpening the image slightly may be necessary.
Any way I started doing all the above, when jpgs and pngs ended up on websites looking terrible. The clients would blame me, or other people would ask them, "Who made those graphics for you? They look terrible!" Providing them with such a pdf with instructions, in my opinion, is the least trouble free method.